
Puppies at this age do NOT have aggression. Puppies play with their mouths, and he doesn't know that he can't play with young children this way.
First - children are not left alone with a puppy of any breed. They need total supervision. Pups leaving mum this early have missed out on playing with their siblings. All done with the mouth - biting. Human babies automatically put new objects in their mouths.
Leaving mum far too early means she has not been able to start his experience of bite inhibition. You say that he won't let go. He does not understand NO. Why would he? You have not trained the leave command, and he doesn't talk human. In fairness, I would have never picked up a pup this young, whatever the backyard breeder had to say about others wanting the pup.
However, there are an awful lot of reputable breeders who would refuse you a puppy with 3 children so young. You must have your work cut out looking after them, and you have a puppy that needs one on one training.
When your pup is old enough, make sure that you get him vaccinated and microchipped. Then join a reputable training class aimed at people like you who have very little canine knowledge. You will need to be consistent with your training.
I think one of your first jobs is to read Ian Dunbar's article "The bite starts here." Google it, you will easily find it. Remember that, if the children get excitable and high pitched, The pup will think that is an invitation to play. And play means biting. So remind the children that they will have to stay calm around the pup.
Good Luck. :-)