CycleSprog has a wealth of info on kids bikes e.g. http://www.cyclesprog.co.uk/bikes/mountain-bikes-for-kids/best-kids-26-wheel-mountain-bikes-2/
For ‘light’ MTB duty a rigid MTB will be a good cheap and light option… I’d second molgrips recommendation of the Pinnacle Kauri: one of the girls at our School MTB Club has one and it’s a very well thought out bit of kit, well up to Blue Trails and XC MTB riding.
https://youtu.be/fmRyEub4yIo (Isla 24, Pinnacle, then some sort of Univega)
If you do want to go down the hardtail route then it’s a bit more of a minefield as cheap suspension is worse than no suspension. My recommendations here would be the Vitus Nucleus 26* (when it’s back in for Xmas – I hope) and the Whyte 403, bit of which we have in our Club.
* I work for WiggleCRC and Vitus is one of our Brands – but I think they have nailed the affordable kid’s hardtail with the Nucleus 26.