My advice would be don’t go too mad price wise. In my case the 24s really didn’t last long before the kids outgrew them.
When daughter was 10, I ended up buying a Merlin Malt frame in small + some REBAs that were on special offer. A few odd bits of finishing kit + what I had in my shed, and we set about building her new bike together.
Was a fun exercise showing her how it all goes together, so she learnt a lot about the mechanics of bikes, and as I had some decent kit lying around it ended up being pretty light. In the end it was much cheaper spec for spec than a complete bike due to my bike part hoarding skills.
At 13, she’s not far off it being too small again, so I might end up buying another frame at some point, and swapping the bits over. Bound to be some bargain 26er frames coming up soon 😉
Si