There may be something here, but I don't know.

Go to the Mozilla Add-ons Web Page {web link} (There’s a lot of good stuff here) and search for what you want.

You can't prevent your children from deleting the history.

There are a lot of ways to clear personal data, either via "Clear Recent History" or by deleting the history and cookies even if Private Browsing mode isn't used to prevent saving this data in the first place. If you want to see which sites have been visited then you should look at others means outside Firefox like a router or firewall with a password protected logging feature that can't be bypassed or a program like Wireshark to log internet access.