Short cranks are definately a good call – will make a massive difference (sorry)!
I've got a 14" Barracuda Subway (sold as Blackjack, Cardwhore and some other names) that comes in very low but not overly short.
Bunch of parts to help the build if you'd like too.
In Manchester, but I have a big box & friendly postman!
I'd think it's worth £50+postage, but I'll do you a lot of extras very cheap! 😉
Could even build it up if you like (I can't resist a project!), have wheels, tyres, stem, saddle and so on, and someone's selling some RS Dart 1s on here very cheap…
Have some 24" wheels too, but they're pretty beefy – probably not what I'd recommend for a youngster!
Let me know what you think,
Pete