If you have access to a barefoot boom there is no better tool for the kids. My 8 y/o has been slaloming for 3 seasons now. I think an aggressive 3.5 to 4 y/o can definitely deep water start slalom if you give them a ton of boom time first. We never did the 2 ski's thing since I don't think it really translates well to slalom or wake boarding. I like to start the kids on a wake skate on the boom. From there it is an easy transition to deepwater wakeboard starts.
Frankly I think the best thing for the 3 y/o crowd is the kneeboard. Super ez to get up on with the hook and they learn how to edge, cut and take a face full of water at relatively low speeds. Get two so someone can coach/help out the really little ones.
Anyways, make it fun for them and get them started early. Reward them for any progression whether it be just jumping into the lake by themselves, trying to do something x number of times, etc. I bribed mine with slushees if they gave anything a few tries...it's amazing how much fear can be overcome with baby steps, a lot of patience and a slushee. Here are a few pics from of my kid.