I'd recommend two steps in getting a kid into a bigger wake like the X25.
First start them off on a long rope 75-80'. Tell them to stay in between the wakes. Get them comfortable going straight first. Then have them go back and forth between the wakes. As you feel they have some balance and board control - probably at least several sets or days depending on the kid, age, prior experience with similar sports, etc - you can start increasing the boat speed. I don't get mine above 15mph until they look very confident and are easily doing S-turns between the wakes. Once they get up to 17 or maybe 18 mph it's time to make it more challenging.
Step two, I change them to a very short rope - 55' or so - and dump all ballast on the boat. The combination of the short rope and unweighted x25 will give them a clean wake to play on at a low enough speed they don't injured themselves and/or start to get scared of what could happen. Then I try to have them go outside the wake then come back to the middle in each direction at the lowest speed I can give them a clean wake - will depend on the boat, rope length, and how many people are in the boat but hopefully you can do it in the 15-17mph range. Let them get used to dealing with the wake without jumping yet.
I had to switch my coaching technique when we moved to an X25 from an x15 because of the bigger wake. That's when I changed to a long rope to start and make them learn in between the wakes before shoving them on a big wake. Even unweighted, the x25 wake is good sized to a 4' and 60lb kid. The bigger wake also doesn't clean up at as low a speed as boats with smaller wakes. I'm a big believer that you want to teach kids at as slow a speed as you can so they learn to love the sport and not fear it. Drag them into a face plant at 18mph is a quick way to get them uninterested in going again.