I have found that there are very few kids of age 3, 4 and even 5, that can learn things such as forms or specific techniques and understand them. However, it is not out of the question to take those same children and work them on basics. Punching, kicking, blocking, stances and coordination drills. Don't get me wrong, it's not ALL kids, just a rule of thumb.
Working these kids on basics until age 6, then moving them up into the next class level, can be very benficial to the child. And what the heck, why not make that advancement a birthday present from the school on their 6th. Once at the next class level, they know all of the basic movements, they just need to then begin putting them into proper sequences. I am able to verbally ask a student to turn 90d into a cat stance, facing 9 o'clock and they do it, without question. Now step out into a left fighting stance and use a #8 block, they do it. Makes life a bit easier.
Oh yes, there is the once in a while parent that believes their child is a prodigy, and some may very well be, but knowing basics inside and out benefits them more the regurgitating a form for which they have no idea what they are doing other than mimicking movements of their instructors ... anyway, just my humble opinion.