on-one 456 might be suitable but quite heavy and possibly too long in the top tube.
i can happily recommend a 'dual' frame from merlin: nice and teeny, really light and builds in to a brilliant little ripper. i did this for my daughter two years ago. she has since had a bit of a growth spurt and moved on to a medium cannondale but its adapted really well. from being 'just' bigger than her specialized 24" wheel up to 'nearly as big' as her medium cannnondale with just stem, bar, seatpost and seat changes as she has grown. you can get a real bargain on them, about £70 quid i think.
im actually selling my daughters if you are interested. havent got pics just yet as im in the middle of last minute sorting for a birthday party but if you drop me an email at kingtomtom@hotmail.co.uk ill sort you some out. its a nice spec; deore/lx/x7, mavic wheels, hayes brakes, magura forks… even got a carbon monkeylight bar on it!
tom