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What bb and crank setup is on the current bike, or are you shopping around for one now?
Pictures might help. I'm guessing shorter bb axle isn't an option?
I might have a some child specific cranks knocking about if that's a workable solution, I can have a look next week when I'm at the garage
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I think you have hit on a big problem for current small persons' bikes.
This is especially the case where the bike imitates the style of a moutain bike (and other 'modern' styles) with a high bottom bracket and long cranks. It's pretty common to see kids' bikes with the saddle flat down so the child can reach the ground and cranks so long the almost come up to the saddle. No wonder so many adults reject the idea of cycling as a means of transport if their early experience of it was hopelessly uncomfortable.
The Trusty shown above has a reasonably low bracket and sensible length cranks. The BB is a standard british cottered setup so the Q factor isn't too bad, although in an ideal world I'd like to make it narrower.
I realise this is only of limited help since you can't go out and find another Trusty, but I hope it gives you an idea of something that has worked, and which I hope will go on being useful in the future. I believe this something worth taking trouble over, since once you have a good machine it is likely to get passed on to other later (grand)children.
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psg1ben
clubman
@ffm
Slight crosspost from the Q-factor thread: has anyone worked out a way of building a kid's mountain/hybrid bike with a decently narrow Q-factor without breaking the bank? I tried to get the stance as narrow as possible on mini-m's current bike but because of the daft crank design (only 157mm long but with unnecessary "flare") it ends up in the same ballpark as an adult hybrid i.e., 170-180mm). I've just measured up an Islabike Beinn and they get the q factordown to 146mm which is almost exactly the same as an adult road bike.
Of all the things that would induce me to buy an Islabike it's that they get this right. Once you've forked out for child-specific cranks (Hup), to achieve the same thing, you may as well buy an Islabike because its resale value will mean you end up spending roughly the same in the long run.