This seems obvious, but ATV cleaning begins as soon as you finish riding.
If your kids have been riding heavily in mud and dirt (particularly on a wet day), the ATV will likely be covered in it.
As soon as they get off riding, you’ll want to hose it down, or if you have access to a pressure washer, even better. You want to go for the guts of the ATV first, so underneath hit up the break pads, the break line, the axels, callipers, coils, springs – anything that moves, rotates or compresses, you want to get that mud off immediately before it dries as this will make the rest of the ATV cleaning process more difficult later on if you leave it.
The plastics aren’t difficult to wash off, but you’ll want to do this too and hose them down at the same time.
The ATV will still be dirty – it’s impossible to get it perfectly clean with the initial spray down, but this first step of cleaning it straight after will get rid of most of the mud.
Ideally you’ll want to do this on site if you can, as it gets messy! If at home, do it outside near drainage so you can wash the excess mud and dirt away quickly.