Not a clue, but someone on my forum has had his Daughters bike nicked, and some of the members are getting together to secretly do a whip round. Problem being what size bike to buy as i cant think what the "Normal" size would be, anyone got a comparison chart somewhere?
Youd want a 20" bike for a 7 year old.... that should do till shes 9/10yrs old for an average child
Look at some of the trek kiddies range,
they have a system of moving many major parts to keep the bike fitting for many years.
The pedals have two positions, and the stem is adjustable in height and reach.
Trouble is, they cost a bit more than your average. The boys version is called an MT220, not sure about the girls.
*edit*
http://www.trekbike.co.uk/2007/bikes.php?sec=overview&MainCatID=6&SubCatID=&RangeID=&ID=289