considering they've all ridden a YZ 80 and you say they loved it, i would be going a full on race bike for each of them.
the 7 year old i would be looking at a KX/RM 65 (same bike, different colors) not as fast as the KTM 65, but reliable, will last him until he's ready to move up to an 85.
the 12 years old i would be looking at an 85 big wheel, or even a super mini, if you look at an 85 BW, i'd be looking at either KTM (expensive, fast) or RM (reliable, easier to ride to rider, cheaper).
then for the 14, a 125 is the obvious step, you could go RM again and all be on the same color (helps with there being no arguments..) but the RM 125 is quite a peaky, agressive 125, but handle supurbly, i personally like the YZ 125 the best.