Restart in Windows without changing startup disk or having to use administrator password?
BootChamp requires you to input the administrator password at its first use, but after that it allows you to restart without asking you for the password again. Unless you child is tech-savvy enough to get into Keychain, I think BootChamp may be exactly what you're looking for. It's also very easy on system resources.

After installation, you could reboot into Windows yourself, giving BootChamp future permission by inputting your admin password. From then on he'll be able to change OSs without having admin access.

I have had BootChamp crash a couple of times, usually when I plug in a USB device or remove one without using OS X's remove function. I've been running it for several months with only five or so instances, so it's pretty rare, but something to consider.

Cheers!