I get the kids floating with the board and rope and tug them by hand. Usually they float for a few minutes and are done. I try and teach them how the board floats and to relax, then when they feel the tug push against it and try to keep their butts in the water. After a while they are ready to be tugged harder, the younger the better because you can pretty much pull them up by hand. After a week/month/day (they all progress differently) they are ready to be pulled over so they feel a face plant, this is the most important step, as you want them to see they will float and get used to holding their head up, if they are very young they most likely can't flip the board back over under their own power and will have to wait for the boat to come around.
You can do all this in a pool too.
To teach them to flip the board have them straighten out as long as possible and roll hard, again very tough for kids six and under. All this applies to wake boarding but I noticed a few boarders joined the thread..good luck!
Here's my 4 year old: