A board they can stand on. A small foil that won’t buck them off, placed back in the box. Tow them slowly.
My son did good on an efoil, but he would just lay on his belly and ride it like superman. Til he got a face full of water after breeching on a cold day and was over it after that.
I got both of my boys (9 and 12) up foiling behind the boat this summer in about 10 pulls each. They already had wake surfing experience, so they knew how to stand and balance on a sinking wake surf board, which helped a lot.
I put the 12 year old on the Armstrong Evo 115 and the 9 year old on the Progression 100, but either foil would have worked. The Afterburner tail added a lot of stability. I positioned the mast pretty far back, better to be slightly under lifted than over lifted during the learning phase.
The biggest factor was the board, the Armstrong WKT. It is a true sinker. With the boat idling, they could sink and balance the board, rise slowly onto plane, and edge out of the wake. Once they were stable, I gradually increased speed and let them come up on foil when they were ready. Use a longer rope to minimize turbulence and allow them to avoid the wake.
This method is the opposite of the big board and big foil approach. I have taught plenty of adults that way, and there is always wasted time fumbling to the knees and then getting yanked by an oversized foil. Going forward, I am teaching with a sinker and a smaller foil every time. Stability comes from speed and direction, and big boards and huge foils work against that.
Good luck. It’s pretty magical to share foiling with the kids.