Puppy playing or aggression?

Labs and Hounds generally have good temperments. My girls growls even now when we play tug of war and barks at me when we play chase and she is a 62 pound pitty, I know her limits and she knows what is acceptable but it did take some learning and injuries (her feelings and my skin) on both our parts to establish these guidelines.
Naturally she plays rougher then yours but labs are known to have what they call "soft mouths" I advise you google that one and read up on it. She is still pertty young and this is the time when her litter mates would teach eachother manners. They learn from biting one another that it hurts. If she is actually biting it is imperative that you let out a loud "yelp" and momentarily turn your attention away from her. Meaning like 30 seconds because her attention span is tiny at this time. I suggest teaching her "no bite" or "gentle"and giving her alternatives to chew on. Watch her play with your son and reprimand when necessary. She'll get the hang of it but it dosnt sound like she is being aggressive at all, she just needs some direction! Congrats on your new fur baby. Wow a 3yr old and a puppy, you must really eat your wheaties, ha ha! But now your child will have a best friend for life!

It sounds like her prey drive the reason for her to bite at your hand. Some breeds have more prey drive in them then other breeds.Puppies growl. Even when they play with their litter mates they growl at one another.

i also would say that some dogs are just more "vocal" than others.  my one dog seems to have a sound/noise/growl for everything...playing, begging, whatever.  my other dog is not like that...she doesn't even really growl when playing rough.  it was strange because i had the vocal dog first, and then when we got the non-vocal dog, it struck me as weird that she wasn't making noises for every little thing.  but the one that is vocal is not aggressive at all, and i'd say of the two, she's less aggressive.  

go watch puppies play with each other. What do they do-- they growl and bite each other. It's not aggression. It's also not an appropriate way to play with a human. Pups need to be taught how to properly play with humans. Best way is to use a toy as an intermediary. Get the dog to direct the biting and chasing towards toys. Tug of war and fetch are probably your best bet. Get a tug toy, and run away from the pup, and encourage the pup to chase after and bite at the toy, then you can play some fun tug of war. Tug of war does not cause aggression. Your pup may growl loudly while playing tug of war, that's normal.

Just an observation here; you said she's got some hound in her, alot of hounds have a natural instinct to bark when they chase their prey.  It's called giving tongue.  It was used to help the hunters follow their dogs through the brush and woods.  What you're experiencing may just be the precursor to her barking.  If it is instinctive behavior and you don't like it, training it out of her may become very difficult.  If there is any mouthing or biting during this play, you need to end the game NOW!  Hounds love to play and once you establish the rules they generally will abide by them.  Good luck!