I wouldn't dump too much money into the 24" bikes. I gave my nephew a Fuji Ace 24, and there are many things that are very cheap on it. Anyway, it had an indexed paddle shifter on the tops of the bars. It works reasonably well.

Sora 8 spd shifters are available.
Shimano Sora ST-3300/33003 3x8-Speed Dual Control Shift/Brake Lever Set Bikewagon

Does your bike use a freewheel or a cassette? You can, of course, buy 8 speed freewheels, or even 9 speed freewheels (which would open up a lot of possibilities).

I've snagged a couple of bikes that came with Claris 2300 shifters. They're ok. I got used to the thumb lever fairly quickly.

However it may be easier for your kids to learn with whatever is on their bikes now, then get better shifters with the next bike once they are bigger, stronger, and more coordinated. It never hurts to have practice with a variety of technologies too.