I am an instructor and the mtn where I work just started offering snowboard lessons for 4-6 year olds. In my experiences with these lessons, there are a few things to take into consideration. First, is overall coordination. Once you strap these kids in there balance is usually pretty good allowing them to do fine while going straight. However, the more complex muscle movements needed to twist/tilt the board to make it turn are very difficult for most children at this age.
The other things to take into consideration are attention span and energy levels. Both influence their ability to learn and determine how much fun they are having.
So, I think its a great idea to get your kids out there at an early age just dont expect them to be ripping down the mountain at the age of 3 or 4.
However, you do get the occasional 3/4/5 year olds that can pick up skiing and can advance to green and maybe even blue runs.