I took a friend and his 6 year old daughter flying a while back in a PA28.
Helen wanted to sit in the front next to me, but we decided that they should both sit in the back. A wise move, because within 15 minutes Helen was feeling queasy - she'd a history of being car sick, so we weren't too surprised. Her dad was able to deal with her OK while I concentrated on flying the plane.
But it would have been a problem if I'd had to deal with her, fly the plane and work out whether to continue as planned or divert/turn back. The stress levels can go up very quickly!
Maybe the younger they start, the better chance you have of a trouble free trip! Good luck!