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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
I think this is pretty key. A quick eyeballing of that Islabike suggests that there is about 40mm of BB drop. As far as I can see on mountain bikes there is barely any drop at all. Therefore, to allow a child to get their foot down when they need to they're going to be 40mm closer to the pedals, which means they're not going to be able to fully extend their leg and it will come up higher at the top of the crank rotation. At a normal (adult) q factor, that's really unergonomic.
ffm
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.