My golden puppy doesnt like friends one yr old?

so i hav a golden retriever that will be one next month.hes an extrememly good puppy and i am sooo proud of him.i have two small kids ages 4 and two and we have had him since january this year.he never once has bit my kis or anyone but about a monthe ago my friend came over with her kids .he daughter who is one was picking between his fingers and he all of the sudden bit her.not to hard just that little warning bite but still it freaked me out.i apologized like crazy and put him in another room.so we umed she hurt him some how.well they came over again the other day and he was fine the whole time she was here but later on he was laying down and my son was hugging around his neck and her daughter was petting his back and he tried to bite her again!!!he didnt leave any marks but still i have no idea why hes doing this.my kids can be pretty ruff with him and he never does this.we play with him all day.he is basically a third child.now i dont know if i can trust him around other kids.he doesnt just come out a bite them but still this is so weird.we have kids around constanly and he has never done this!!!!its like he doesnt like this one girl in particular for whatever reason!!!!!!!!he freaks out by new men too.not that we have them around very much but he gets really scary when someone new is around!he is ver well mannered calm and listens very well!!!so thi is very out of character..please help.!