My friend is giving away his Dogs and I need input

I've decided to take the dogs, due to him having a problem that he can't no longer take care of them. I was wondering they both a yr old, and he basically took care of them since they were born. My concern is, how long will it take for the dogs to get use to me? one is a rottweiler mix with german shepherd and the other is a pitbull mix with labrador. I'm kind of being hesitant by taking them cause I have a 3 yr old son and I'm worry for his safety. any input? I have to make up my mind before tomorrow