There are now two 14" Islabikes, this http://www.islabikes.co.uk/products/bikes/item/cnoc-14-large and a smaller one. Almost all secondhand Cnoc 14" bikes will be this size.
The site has good info on sizing.
Looked after, secondhand Islabikes can expect to sell for much the same price even after another year.
Don't be tempted to get any bike for a child that's too big.
Cnocs have reasonable long seatposts and also riser bars that can be adjusted for reach. Newer ones have aheadsets so don't have the simple vertical adjustment option with 'normal' stem. Some early ones chipped badly, which may put some people off - but usually reflected in price. Some early ones don't have two cantilever brakes, which may or may not bother purchasers and/or young users.
I think my grandson is fairly average height. He got a 14" for his 3rd birthday and a 16" for his 4th.
For a smaller child, new to cycling, a secondhand 14" makes more sense. Resell when grown out of (before reaching maximum possible adjustments if that realistically makes the bike too small).
Frogs may or may not be as well made/specced and hold price as well, but I have no direct experience.