Last year DD1 (one month shy of 6) was allowed to go on whatever rides she wanted with me or DW. We got her onto BTMR, as a test, and she felt it was too fast, so at that point, it was her own choice. Funny though, pretty much each day she would go onto most coasters and even ToT. Granted, she cried each time and never wanted to ride them again, but yet....she tried each one, I was proud of her, for facing her fears. Usually, she'd make the decision while one parent was riding. When it came time for Everest, she'd been freaked by BTMR, Space Mountain and ToT. When I came off, she had decided to ride with DW, and sure enough, she was crying in the end, scared. (We had watched videos of the ridefor months before hand too). I bought hera little yeti for her bravery, and thought that was that. Well, while we were talking last week about the upcomming trip, she wants to ride them all again!! Even ToT, which floored me.
Ultimately, it's all up to the child. I hope at least they will give it a try, and if it's too much, that's okay, facing fears is a part of growing up after all.