Sorry, going to be boring here…
Honestly, for most use cases the cheapest and best option is a used Islabike Beinn 20. Buy a used one for around £200, keep it for a couple of years and sell it again for around £200 (more, even, if the prices keep rising).
If you can, get the later generation bike – the one with the narrower saddle – as it has numerous improvements and will be easier to sell when you inevitably grow out of it. Parts are cheaply and easily available directly from Islabikes and the bikes are absolutely excellent.
I did say that it wasn’t very exciting, but with the still incredible resale values of used Islabike and never-ending demand, any other kids bike will almost certainly work out more expensive over the long term.