It’ll depend on the size of the cassette. The bigger the sprockets, the bigger the difference. The lower end steel cassettes often run 12-36 sprockets. I would be surprised if it was more than a couple of hundred grams difference.

I’m not sure about more recent stuff, but I think XT 9 speed used to only run to a 34 tooth sprocket. The low-end stuff is steel, so it’s very durable. I generally run steel Deore stuff on my bikes because it’s cheap and lasts for years.

Not sure it’s worth the cost buying but if you have some “proper” garage parts that look ok then swapping them in can work. I dropped a load of weight on my sons 24” by putting old narrow carbon bars, carbon seat post, decent stem, pedals and saddle. Those items tend to be like ships anchors on kids bikes.

Any other ideas on how to lighten it?

Tyres

Will he notice?

No.

Difference between cassettes with the same ratios is more like 30g, Halfords page says the bike weighs over 16kg.

You may find that a wider range cassette (which may then require a better rear mech) means he can ride up stuff he’d otherwise have to push, which won’t be much fun at 16kg.

+1 for tyres. Conti Explorer and Schwalbe Black Jack are good knobblies otherwise perhaps Marathon HS420 or Maxxis DTH.

https://www.schwalbe.com/en/road-reader/schwalbe-one-tube-type
There’s a ~195g 24×0.9″ variant, saving ~600g for the pair. 😉

https://www.schwalbe.com/en/mtb-reader/billy-bonkers are ~440g for 24×2″, no idea if that’s the lightest 24″ mtb/tour tyre they do.

It’s just over 12kg hence my thinking small changes could help a bit.

Lots of little changes would.

Things like bar/stem/tyres/seatpost etc will yield you much bigger savings in general.

It’s just over 12kg

That’s a relief!

TBH I’d not spend any money on it bar tyres (if the stock ones are poor). Just let him have some fun on it and you can perhaps put money aside for a nice bike when he gets either too big or if it’s obviously holding back his riding.