Re: How do make ubuntu (grub) hidden?
Why don't you show them what to do when the GRUB menu appears. My 8 year old daughter tri-boots XP, 7 and Ubuntu 10.04, on her own machine-not mine. She knows how to choose what OS she wants to use. It is not too difficult to use the arrow key to select an OS and hit Enter.
You might not want to do that though- one day they may get curious and boot to Ubuntu and like it when they see how fast it is compared to windows.
Multi-boot: Windows 8, Ubuntu 12.10, Ubuntu Development Release Raring & Sabayon X