If you can wait a week or so the new Vitus Nucleus 24 would be worth a look – air fork, hydro discs, and even decent tyres. The list price is £399, but Wiggle do ‘Loyalty Discounts’ so if you’ve bought enough stuff from Wiggle to be a Platinum customer it will be £350 ish.
http://www.wiggle.co.uk/vitus-nucleus-24-kids-mountain-bike-2019-altus/
{for the sake of transparency, I work for WiggleCRC, but I also run a kid’s MTB Club so I’m very interested in quality kids bikes}
… for the sake of Balance I recommend parents look at
Islabike
Frog
Orbea
Hoy
http://www.wiggle.co.uk/vitus-24-kids-bike/
Vitus also do a 20+ bike which is a similar size
http://www.wiggle.co.uk/vitus-20-kids-bike-1/
and I like the look of the new GO Outdoors offering
https://www.gooutdoors.co.uk/wild-bikes-24-kids-bike-p454082