[grugdev42]

I assume you are concerned with keeping your children safe? If so, whether the computer has an internet connection is much more important than the OS.

I would rather have my children use Windows offline, rather than Linux online. There is simply too much bad stuff out there regardless of the OS.

Another thing to consider is what their school uses. If you want to give them an advantage at school, use the same OS. They will already be more familiar with it!