Don't they get wider where the pedal hole is ?
If you drill them further down, you'll be making a hole in the narrower part which will weaken them.
How big and strong is the boy ?
There must be a safety margin, if you're shortening the cranks because he's only little, they probably won't break.

I doubt it as this will affect the strength of the crank too much, but it might depend on the crank

It's a very thick set off a downhill bike. I recon I could drill lots of extra holes to lighten, and I wouldn't snap them.

I seem to remember there was a company offering a crank shortening service, so it can be done but they could only do solid cranks.
EDIT- here you go, http://www.highpath.net/highpath/cycles/shorten.html

Nice, thanks.

And if Chris can't do it, I also do it:

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fwiw trek kids bikes (well, my kids 20" ones in any case) come with cranks with 2 holes in each to move the pedals further down the crank as child grows. The arms are alloy too.

Also/but, riding with even a not-very-bent crank is most peculiar: (anyone want an old and lightly bent fsa megaexo crank/bb?) -you'll need to get the location and angle of the drilling absolutley bob-on or they will ride 'bent'.

Ben, any tips will be welcome.

I use a special jig in a milling machine to make sure I get the new hole exactly parallel to the old one, then good quality taps.

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