Depends on the kit and style of music. How big is the kids kit, and what style of music is it?

If it's a relatively smaller kit, 4 or 5 peice, then you can get away with 4 mics, but thats only if he doesn't do a lot of fills and its not metal.

I say you just keep it 4 mics since it would complicate you to have more. Your setup will be like this:

Bass drum
Snare
Overhead left
Overhead right

For the bass drum, get the mic inside of the drum, which will be easy if he has a hole in his front head, if not, have him take the front head off and place the mic about 3 inches from the center of the beater head where the beater strikes. See if he can play with the front head off, if so, just leave it off. I really prefer the sound of a bass drum on recordings without the front head, but I personally can't play with the front head off of my bass drum. Theres just no air to bounce back so it makes it more difficult to get double strokes and such.

For the snare, its all about experimenting here. Most people usually mic the top head, just slant the mic and position it the way you'd imagine it, it's not very difficult. But I suggest trying the bottom of the snare and seeing if you like that sound, not in the center, but directly pointing up 1/4 of the way from the edge, I really like the sound I get out of this, on my Premiere snare atleast.

For the overheads, I'd say, hmm, 2-4ft above the cymbals. Since you're not micing the toms or the hi hat here, just use your best judgement at positioning them to get the best mix of cymbals/hihat/ and toms. The only rule is to make sure they are both of equal distance from the snare. Just sit down on the throne centered and hold your arms up spread apart to where you think its pretty equal, go ahead and measure it if you want. Just use common sense. Pan the left overhead to the left channel about 75%, same with the right in the right channel...just mix to what you prefer.

Also, and of course, you can never stress enough how much drums that are tuned good is important. Plus, I don't know the kit, but if he doesn't have that great of cymbals then that could easilly ruin everything. B8's or ZBT's just don't cut it on recordings.

The real chunk of the cake here though is if this drummer can work his accents good for this recording. Remind him that he doesn't need to beat the crap out of his cymbals for the recording, a lot of drummers play excessively hard. He can easilly make or break the recording himself, by how he plays his kit. He'll need to play his toms, ride cymbal, and closed hi hats much harder then the crashes, and lay off the snare.

Good luck.