As the dad of a four year old who uses flash in Ubuntu Linux, I have to ask, what distro/version are you running?
In 9.04 (and for the last few releases as I recall), when Ubuntu encounters a flash file in Firefox for the first time, it *offers* to install Flash.
When you set up the system, you could install the flashplugin-installer package which downloads and installs it for you. If you enabled the Canonical Partner repository when you set it up, you can install adobe-flashplugin and then its all done.
Xvid would require a codec download; similar to the flash plugin, Ubuntu will offer to do that when you try to run an Xvid-encoded video file without the codec installed; alternately just installing a big batch of gstreamer plugins when you first set up the system will ensure you don't have to install codecs later (I would recommend: gstreamer0.10-ffmpeg, gstreamer0.10-plugins-bad, gstreamer0.10-plugins-bad-multiverse, gstreamer0.10-plugins-ugly, gstreamer0.10-plugins-ugly-multiverse).
DVDs are almost all encrypted and its a little more work; you need to add a suitable repository (Medibuntu) and install the libdvdcss2 package.
Now, if you can't be bothered to act as the OEM for your sons or invest a couple minutes figuring out how to help them, that's one thing, but this isn't 1999; Linux in general, and Ubuntu specifically, is easy to use and there's quite a lot of help available, e.g. http://ubuntuforums.org/