How do you know what size bike to get for a child?

[Moosy]

On the Halfords site they're sized by age but I don't know if my DD has the right length legs for her age! Also we want it to be a surprise for her birthday so don't really want to take her into the shop to try them out.

Is there a rule about bike size/inside leg measurement that I could work from?

If you look on the Argos website they show the leg length.

Perfect, thank you!

So when I measure her leg do I go from the floor or from her ankle?

I'd say floor. We got a bike for Dd a couple of years ago, she was 9. She has very short legs for her age, when we went to the bike shop we were advised to get her a 24" wheel. I was but the owner said to get a low frame one, but with the bigger wheels, she has got 2 years out of it so far.

DD1 is about to be 4 and going by her height Argos/Halfords seem to agree that she needs a 12" wheel, but I'll measure her legs just to make sure as I think she looks quite long legged.

Sometimes it helps to see the bike,as well. Good luck