I have been with my niece twice while she had her hair braided, she is older though (was 10 and 13 at the time), and it took a long time. Her hair is thick and long, so short hair would obviously take less time. She did say it hurt a bit (pulled tight), and the last time she said it wasn't as bad as the first.
The two things I can think of are: (1) getting a sunburn on her scalp, and (2) not being able to sit still long enough to finish, and ending up with half a head braided.
I think you can leave them in for a couple of weeks.
HTH, and have fun!
I recognize you from the pp boards, we ended up with a southern caribbean cruise with our free airline tickets, maybe someday we'll make it to Hawaii.......