If you kids can ride bicycles they can ride dirt bikes, ATV's are just plan easy to ride. Really comes down to preference and personality. I've had both and prefer my ATV's over the Dirt bikes but always had fun hill climbing and jumping stuff on the dirt bike.
ATV's are tricky - there are government regs on how old kids have to be to ride.
ATV's 250cc and up you have to be 16yrs old to operate them.
If they are old enough Check out the TRX250EX and the TRX400EX
There are no restrictions on dirtbikes just have to be able to touch the ground so when you stop you don't fall over. Also if the skill level is not there you kind of want to steer clear of the performance (R model race) machines.
I would check out Honda's line of enduro play machines I would say the CRF150F and the CRF230F would be good machines.
If you have more questions email me or call
rob@hondaeasttoledo.com - 877-437-1631
Later
Rob