Another parental control type question

[AylmerQc]

Somewhat similar situation to the previous post. In this case i 'm letting the kids use the Mac until I get their pc setup on the network. First day on the Mac, looks like my son erased all my Firefox bookmarks. They don't show up when I click on Bookmark and the Bookmark Menu is different, (more options but no bookmarked sites). I can't figure out if they're permanently erased and how he could have done this, wether on purpose or by mistake.
Anyone have a clue...?
Another question: I often read how if we don't install or accept anything we're not sure of that there's little chance we'll get malware on our macs. Is it possible, kids being kids, that they'll install or accept harmful downloads if I haven't given them the  Admin password?