I have 5 small children in my home. I breed a large breed of athletic hound. The number one most important rule of thumb is structure. Let's break this situation down from a very basic viewpoint. If I am saying anything you already know it is not to be patronizing but it is to be as clear as possible. I hate leaving things to chance so please bear with me. Some of this may seem ot but in reality it will help you make a choice about what you need to do to keep this guy in your home.
This is most likely the pup's 4th to 5th home in his brief little life. Begin with the breeder, then first home, shelter or first rescue pull, foster and finally your home. That is a ton to process for the little guy!! The chances of separation anxiety are pretty good. He does not have a solid base and sense of structure in his little head yet. Now ask yourself why did a nice little guy end up in Rescue to begin with?? sigh.... I would offer the probability that it was an impulse buy by the first owner. The "breeder" may have made little effort to really be careful about the placement, or made no effort to remain involved afterwords. The pup may have begun to act like a pup and the new owner had no clue how to correct the problems. There is always the possibility that the new owner fell on very hard times but thad the puppy been bred correctly , microchipped and registered by the breeder it should have gone back to them. Since it did not I makes me question the effort and intent from that person. This pup may have been placed as young as 6 or 7 weeks , all of these little details make a huge difference in the pup's ability to process and grow.
At 7 months the dog is far too young to be left outside unattended for a few hours. This is not an invitation but a guarantee there will be damage. Between boredom and the possible SA it will be up to you to plan his day to avoid the acting out that is more normal than not. He will need an outside kennel area for the time when you can not be with him but you want him outside. ( You see the run type kits at home depot stores) not overly large but secure, covered and you can put toys and chews to entertain him while he is in there. THIS IS NOT your solution it simply allows you additional control and help. Left in there too long he will begging to dig and bark so please do not see this as a fast fix. In the house the dog needs a kennel. Something he can learn is safe and his. Begin with possibly just feeding him in there and increasing time spent. Again This is not your solution it is part of the process to gaining control and being able to work with both the pup and your child.
Children are not a huge problem when you are trying to teach a pup, Pups are not a huge problem when raising a kid (s)..chaos is the undoing factor . Both share levels of unpredictable behavior and needs and since you can't simply schedule it you have to learn how to cope with it. The dog you lost was settled in it's ways and able to read all of you with ease. Your little guy is still trying to learn.
Let's talk Mom Talk about the pup's ability to process..when you were potty training your daughter she learned the word first and knew what it was..then learned to associate what you did there, then she began to understand what you wanted her to do there...eventually she began to use the potty and after a time do so dependably. Your pup is going through much the same thing. When you said
ann404:he seems to follow commands during the day fine, sit, stay.
you say that as if he is trained therefore any misbehavior is a choice. He is not trained, he is getting there but he is in no way "trained". A trained dog does not have time frames to pick and choose when to behave or respond. He is associating several things that you are missing. Things like your nevousness over the new baby coming and the 4 year old's change in behavior. The changes in the house, and even in the short time he has been with you he understands and sees the energy flow and ebb you are experiencing as an expectant mom. He knows when you see his happy face and glowing coat you feel secure and content, then he plays too roughly and you are scared , tired, angry and out of answers... he does not know how to deal with this.
He is not to be out with her until his response and ability to listen is clear and reliable unless he is on a long line. Put him on a gentle leader head halter, Buy a 25 foot legnth of lightweight cording , attatch a clip to his collar's end and have the other end fashioned in to a handle. Every 2-3 feet down the line tie a knot. He is allowed to run and play but should you or your husband see him barrel towards her the rope is shortened and he is corrected by himself because he will hit the end of his alloted room and his own movement and weight will correct him. Change up his legnth of allowed line and use a control word to teach him to slow down... It can be anything you will easily remember I like "Easy" when they begin to get excited and a Stern "Manners" when they are going to act out ... Or even a Mommy noise UH UH !!
The rough housing stops TODAY. Explain to your daughter that you have to be a team to raise this boy because he needs you to be. That she is a very important part of that team. Under no circumstances may he play his biting game. If she allows it she will not be allowed to play with him. PERIOD At 4 she is old enough to understand this. Don't let anyone tell you it is too hard or complicated it is not. Punishment is guaranteed for being rough with a dog. Loss of privileges to time outs , what ever it takes. ( side note we never have a time out etc last longer than the age of the child when they are under 7. so an infraction for a 4 yr old would get a 4 minute time out where you can watch her but she can not interact but has to think) Get her a small bag or bait pouch and out some yummies in it. ( I like goldfish crackers because both kid and pup can share!) have her put the pup through his commands. Get her a brush and put it in her own special bag , have him down and stay an allow her to gently brush him ( not long but always controlled) Explain to her that you really need her help!!! That between the new baby and the puppy you all need to work as a team. You must teach ( insert name) to be easy with babies. He is just a silly puppy and needs you to help him learn. When you work together, this is you and Dad working with her you need to allow her success and help her understand his not learning fast is not because he does not love you all but because he is a baby ... Put up a chart and reward her with stickers for feeding him everyday, ( He must sit first) , helping t pick up his toys... have her put up a chart for him that she get's to award stickers on .... for coming, downing etc... make it tougher as they grow. AND PLEASE do not forget to enthuse wildly and lovingly over the two of them even though you have a hundred million new things to think of each and every moment in this last week or so before the baby arrives and then after when the real chaos starts.
I offer this advice as both a mom and hands on grandmom. I have a large FILLED to the brim home full of people, dogs and action. It is not impossible to make this all work just check out the profile photos of my place I do understand ...... IT will be hard. One last piece of advice.... It is fine to say honestly and openly this is just all too much right now. The timing was not the best and with your older child acting out you may simply not have the energy to deal with everything . Better now , when he is still young enough to place then later when it will become more difficult. Or you have heavily invested big $$ in him. No one will fault you for thinking it all through and making a tough call.
Best of luck with ALL of the adventures in your life.
Bonita of Bwana