You might need quite an old tablet (or possibly one of the several hundred quid pro ones might have a driver) for the drivers to work on the G4, also the operating system will be not so great, much newer software won't work, and a lot of newer hardware will be flaky.
For drawing with a graphics tablet you'd be best off just fitting the tablet to an existing PC. A G4 tower is really a creaking heap of badness, not to mention being incredibly power hungry, and very noisy sometimes (we used to develop with them, and half the time they sounded like a plane taking off). The G4 towers are a big part of the reason they went to Intel processors (the fact they could never make a decent laptop was the other big reason).
There's no massive advantage to an Intel Mac for drawing (potentially some other advantages of a Mac though if you like them), a G4 mac will be actively worse than even a £200 PC, particularly if you want to attach new hardware to it or browse the internet.
For a kid painting & drawing, you want a graphics tablet, and probably want Artrage (http://www.artrage.com/ ) which costs a bit of money (not much - I think it is $20 for an okay version), or potentially mypaint (http://mypaint.intilinux.com/ ), which is free and very powerful, but has quite a steep learning curve for a kid (artrage is dead easy to use).
I've got a wacom bamboo, and it is a pretty good tablet for the price - only has pressure sensing not tilt, but I didn't have £300 to spend on the intuous professional ones just to do a few bits of illustration. I have done a fair bit of work on it (probably 50 hours or so) and it is still great. I have the one with a touch pad on it - that is a waste really, I only got that version because the pen it comes with is better than the most basic one. I think all the wacoms support macs if you must go that way, although whether or not they will support as old an OS as you're likely to find on a G4, or non-intel macs, I don't know.
Joe