I would keep the G400 for yourself (better all-around card, IMHO), and give your son the V3. Two reasons for that - first, and more importantly, the Voodoo is the cheaper and easier to get of the two cards, so if he screws it up, it will be easier to replace.
Secondly, as far as stability and compatibility go, i don't think i've ever seen a better card than the Voodoo series. In performance measures, the card is a couple of generations behind, but it's fast enough for a child - and where the card really shines, is that reliability factor. If i could only pick one card to test systems with for the rest of my life, it would probably be the V3 (well, it already is). I have never once had a problem using a V3 card in any modern PC i've ever tried it in.
Which makes it an ideal card for a child's system. It's cheap, it's relible, it's a reasonably good card for any purpose a child could use it for including gaming, and it just plain works, without hassle or fuss. Save the Matrox for your box
