Start out WAY WAY WAY WAY slower. Your kids will be able to stand up on that board at just above an idle speed. Tell them to keep their butts close to their heels until the board has planed out of the water. A lot of people worry about standing up before the board is up.... Let the boat do the work and then just stand up. Get them up and riding on their heels outside the wake before you increase speed and make it surfable. If they don't get up after 3 tries, break time for them to rest. That's an old water skiing rule.... After 3 tries you'll be too tired to try any more and will only become frustrated.
Once they can stand up on the board: weight on the balls of their feet and their knees bent. Kids have a very high center of gravity and need to really get low to find success. That board is going to be hard for them to steer and control fore/aft, but they should find some success surfing once they find the sweet spot.
Former owner of a 1987 Supra Saltare. Current owner of a Malibu 23LSV.