Dog problems,please help!?

My Mom is at her wits end with one of my dogs. I own two labs, both one year old. One is well rounded; the other has extreme behavior issues. The well-rounded one always obeys commands. On the other hand the misbehaved one almost never follows simple commands, such as; sit. When he does sit, it never lasts. Not to mention his insane biting problem (this goes for both dogs). He gets excited when he sees small children, and as soon as I let him off the leash to go into the backyard (after a walk) he darts for the kid. My Mom says if I can’t straighten him out I have to give him up. What should I do?