Need some advice with 16 week old rott.

First, never attempt to take food from this dog's mouth at this stage.  Contain him when food is present and the child is also present.  Get a copy of "Mine! A Guide to Resource Guarding in Dogs" by Jean Donaldson, and follow all the protocols.  Be aware that a resource guarder may never be completely "cured", so your aim is to see that no resource that is commonly within the dog's environment is special enough for him to guard, and that he learns not to guard from all members of your family.  Do not trust him with guests about, or let him hang with you at Thanksgiving dinner - this is a dog that should be crated when the family is eating.  You don't want a child reaching for a piece of chicken that falls on the floor at the same moment that Fido decides it's his. 
Teach the pup the word "trade" by NOT taking his stuff, but rather, give him a piece of tasty meat in exchange for the object or food item you want to retrieve from him.  At first, you would hold the special item, so the dog only thinks that the leader is sharing - and you take it away by offering a more special piece of meat.  So, you might hold a mostly scraped and hollowed out marrow bone, and offer a piece of garlic roast beef or liverwurst to get it back as you say "trade".  The dog learns that you always have something better for him than the thing he is giving up.  These dogs should have stuff dropped into the food bowl, too.  For example, you walk by and casually drop a piece of cheese into the bowl while he's eating his kibble.  Dog learns...hmmm, human hand coming near my bowl is a GOOD thing.
Get him in to a positive reinforcment training class asap, and continue his training and socialization until adulthood.
No offense intended, but it amazes me that people opt for guardian breeds when they haven't any knowledge of resource guarding or any of the other aggressive behaviors and don't seem to know how to deal with them.  Breeders aren't selective enough about who these dogs go to, and that's why the breeds face BSL - it's just simple ignorance of how to manage them.  I'm glad that you joined the forum.  At least you will have some resources to call upon. []

At this point you need to keep the kid and the dog seperate when food is involved.  One option, kid in high chair while dog is eating;  dog in crate when kid has a snack.
 
In small sessions, kid in high chair, dog on lead.  Have food get from kid to floor with a verbal cue ("get it").  Dog is restrained by leash, food drops,  dog is restrained until not pulling on lead (no pressure), dog hears "get it" and feels leash loosen, dog gets food.
 
Keep the dog crated while kid has the floor and kid in playpen with dog on the floor. Practice obedience next to playpen with food and treats occurring while dog is behaving next to playpen.

PUPPY AND KID DO NOT INTERACT without direct supervision and control (kid is in a safe area, dog is on leash)

Couple Quick Tips....
 
1) Your dog should be fed every meal in his crate from now on (that isn't hand fed).  I would reccomend this of absolutely any dog...but in dogs that have potential for resource guarding it is a must.
 
2) I would start hand feeding this dog to day...atleat one meal each day.  Fill his bowl, have him sit in front of you and feed him on kibble at a time for the whole bowl.
 
3) After you are confident he will readily except hand feeding from you, to your child to hand feed the dog.  Work on teaching the 2 year old training commands and require the dog to offer the commands before the child gives the chow.
 
4) Increase the amount of exercise your dog is getting....playing in the backyard doesn't count at this stage.
 
5) Get enrolled in a puppy class....even if it's at a big box store...get him in something.

Donaldson, Jean[size=3], Mine! A Guide To Resource Guarding In Dogs, 2002
[linkhttp://www.dogwise.com/itemdetails.cfm?ID=DTB740]http://www.dogwise.com/itemdetails.cfm?ID=DTB740[/link]

[font=verdana]Aloff, Brenda[font=verdana][size=3], Canine Body Language: A Photographic Guide Interpreting the Native Language of the Domestic Dog, 11/2005
[linkhttp://tinyurl.com/yrhl99]http://tinyurl.com/yrhl99[/link]

Rugaas, Turid[font=verdana][size=3], On Talking Terms With Dogs: Calming Signals, 2nd edition, 12/2005
[linkhttp://tinyurl.com/2tk7dt]http://tinyurl.com/2tk7dt[/link][/size][/font][/size][/font]

Donaldson, Jean, [font="times new roman"][size=2][/font]The Culture Clash, 1/1997
[linkhttp://tinyurl.com/642uk]http://tinyurl.com/642uk[/link][/size]

A description of NILIF and pack leader info:
[link>http://forum.dog.com/asp/m.asp?m=298804][color=#666600]http://forum.dog.com/asp/m.asp?m=298804[/link] 
[/size][/color][/font][/size][size=2][font=verdana][size=3][font="times new roman"][size=2][font=arial][size=2]
[font=verdana][size=3]See this post for a good list of initial training books: [/size][/font]
[color=#000000][link>http://forum.dog.com/discus/messages/47/101806.html#POST349863]http://forum.dog.com/discus/messages/47/101806.html#POST349863[/link][/size] [/size][/font][/size][/font][/size][/color][/font][/size]
[size=2][font=verdana][size=3]
Critical Periods in a Dog's Life -- Developmental Stages:
[color=#666600][link>http://westwingsess.com/critical_stages.htm]http://westwingsess.com/critical_stages.htm[/link][/size]
[link>http://www.dogclub.co.uk/advice/puppygrowth.php][color=#666600]http://www.dogclub.co.uk/advice/puppygrowth.php[/link]
 
Example resource guarding feeding program:
[linkhttp://forum.dog.com/discus/messages/169/142179.html#POST433669][color=#666600]http://forum.dog.com/discus/messages/169/142179.html#POST433669[/link][/color][/size][/color][/size][/color][/font][/size]