I have done the 24 and 20 in Specialized kids fat bikes.
Here are my observations:
1. They are heavy
2. The specialized rims SUCK (there is no bead shelf, so the bead will drop off to the inside of the rim at anything lower than 9psi)
3. Gearing SUCKS too
On the 24 in bike
-built a new wheelset with Bitex hubs and HED BAD rims. Stock tires work tubeless down to zero PSI now and shaved several pounds of the weight
-changed from stock 9sp 32-34 gearing to 11sp 30-46
-changed mech brakes for cheap hydro
-carbon bar (the stock one is really heavy)
-dropper post
-weight is somewhere around 28 lbs now, I think. Not great, but the kid can get up a hill now.
On the 20 in bike
-changed gearing from 9sp 32-34 to 10sp 30-42
-swapped brakes
-carbon bar
-could never get the wheels to work right and never found a replacement rim that looked promising to run a 4.0 tire on. Running around 2 psi the tire wobbles quit a bit because it partially slides off the rim towards the inside. I could never get it to work tubeless at low pressure either.
-still at 30+ lbs.
-I just use a towee to get the kid to the top of the hill. Going down isn’t a problem.
Overall, I would say the 24 is worth it if you live somewhere with decent snow, as long as you are willing to make some modification. At least wheels and gearing needs work.
The 20 in bike doesn’t work unless you only ride downhill or can somehow figure out the dilemma with the wheels. I think in that size you are better off with a plus bike, especially since the kids weigh so little at that age.
Hope this helps.