I'm going to ditch friends during our vacation due to not wanting to be around their difficult child
Why do you want to ditch them?
My friends expect me to help them out sometimes
It is hard to deal with children when you don't know much about them but it is not okay to ditch friends that have a child. It would be better to work things out otherwise the friendship will end.
Meh, I can live with that. I don't really care about them
I can see the frustration of being forced to do something enjoyable on a vacation.  You should decide on whether this vacation is an ideal scenario for you and talk yo them about it.  It is okay to go and help them or not go if you do not want to, but you should not make them expect one thing and ditch them.