Dawes have some Junior bikes in 24" wheel that look quite reasonable - no idea on availability though.
You mention Islabikes - I own 6 at the moment!! - we have 4 grandchildren (2 daughters, 2 each) and I have provided Islabikes for them from a balance bike through to the 12 year old on a small road-bike. I've bought and sold them as the children have grown into and out of them (I have 2 in "stock"!)
If you can finance an Islabike my rule of thumb is that you'll get at least 70% of what you paid back again, if they're looked after, when you sell them. All of mine have been bought secondhand/sold either through friends or ebay.