Once they have learned how to make alpine turns in a variety of conditions, climbed a few hills in AT gear and seen the ski partners having a ball telemarking, they will be ripe for the next step. Unfortunately, the feet are still growing fast and it will be tough to keep a pair of boots for more then one season. Shop the sales and the swaps, get the best boots you can find and afford. Make your compromises with skis and bindings, but get good boots. You will be surprised how much money you can recover when you go to sell modern tele boots after a seasons use.
Plastic boots can hold their value quite well.
With all the fundamentals of skiing in place, the actual learning to telemark should be no problem. Bringing your kids along the path to telemark bliss will be the most rewarding part of the journey.