Taught my 4 year-old to ski off a 5' section off the boom, just to get him used to the pull up. He was very short and light for his age at the time. We used my kids skis from the early 1980s and we just tied them together so they couldn't spread more than his shoulder width.
Once he mastered the start, we would let him ride for a bit, then slow down to a stop, and pull him up again and again. After about 2-3 days off the boom, he went behind the boat at about 28' off length and he popped right up. Never went back to the boom.
Kids progress fast. Within a few weeks, we untied the back of the skis, and then a few weeks later, we untied the front.
The next year, we did the same process to teach him to slalom ski, and within a few weeks again, he was back behind the boat.
Now, at age 6, all he wants to do is barefoot. I'll keep him off the boom for a while there.
Kids are pretty resilient...biggest challenge is managing their expectations and trying to remain positive while they're telling you you have no idea how to drive a boat!