I started sharing an email account with my step-father when I was six, and within three or four months I had my own account. My knowledge of computers was very basic, but I could clear the cache, cookies, browser history, install and uninstall programs, etc.
I turned out okay. :) Relax, guys. I had no idea what the alt.sex people were talking about until I was ready to know what was going on. Porn was moderately alarming, but something I was going to be exposed to eventually no matter what.
The Internet is a big wide place, but they'll find their peers--What are you afraid of? That they'll become enamored with a different distribution of Linux than you? :) You raised them well. Tell them not to give out personal, physical world information, and let them go.