Or does it have to be a specific 24" wheel Fork- is there such a thing?

There are 24" forks, but I've not seen a decent one.

Two of my lads run 24" wheels, both use normal/26" forks – just means they have a bit more mud-room.

yes, need a 24" fork. They are available but tend to be low end stuff for kids bikes or very firmly sprung 'jumpy' things.

I think Scott and Kona both do some fairly high end 24" wheel kids bikes so might be worth a look to see what they use.

yes, need a 24" fork.

Why?

coz otherwise it'l ride like a chopper 🙄

But if you spec a 80mm travel will that not be ok – like fitting a 100mm
Or fit a travel adjust fork?

No.
the bike will have been designed for a rigid fork, most kids 24" bikes with suspension have between 40-60mm travel fork, not a 80mm fork designed for a 2" larger wheel.
not that hard to get your head round surely?

measure the axle to crown onthe existing fork – it's likely to be only 380mm or less. Addign 60+ mm of fork to the front end will bugger up the handling no end.

not sure what your kid is riding but unless it's pretty rough most kids are better with a lightweight rigid fork than a heavy suspension one their weight barely moves.

You can get a fork to work pretty well for kids even on kids 20" bikes with a bit of ingenuity.