okay, I'm going to compare (hope that's okay, I'm happy that you're happy, MilaM, just want to see). And keep in mind I'm all for cheap and cheerful bikes that are fit for purpose. The Giant is considerably dearer than OP wanted, but what the heck...
Both 20" wheels, Giant comes with suspension, Beinn doesn't.
Islabike Beinn 20 Large:Not so cool paint job, overwhelmingly Al & chromoly parts; 6 speed, lots of SRAM & Shimano parts, £249.99; 8.8kg.
Giant MTX 125: Cool paint job, Mila paid under £200 but tending to cost 220-250 now, but always worthwhile to scour for a bargain price; mostly Al frame & wheels, 6 speed, Shimano rear derailleur, 12.22 kg.
It's the weight, especially...the buyer pays an extra 50 quid to shave over 3kg off the weight. 3.4kg might not seem much, but for a child who only weighs 16-25kg themselves, it's a huge difference, especially if you want to go out several miles from home & back.
Keep in mind, OP, you pay 100 quid less you can get something like this. Overwhelmingly steel, not the best quality components, over-large break levers & too long cranks, lousy mudguards and maybe 14kg weight. It just won't be that pleasant or easy to ride. That's why I suggest a carboot bike, if you won't have that great a bike why pay much for it? I've been quite happy with mine.