Flat ground is best for learning to ski no matter what age. Long lengths of flat ground is better. Availability of different locations close to home or close to you may make it worth thinking through where else in the world that you can visit to go skiing for the first time or for a short / long period of time where you can practice and get better before then accessing the close to home resorts/ ski fields. For those families who travel to America it is worth looking at the small ski fields in lesser travelled states. These have fewer people and cheaper rates, allowing days where its only a partial ski day to not be as frustrating or as expensive. Look up cheapest resorts as a search term in Google or Bing.
Small areas in open field or back yard can be created into obstacle courses familiar for kids and teenagers attending indoor Occupational Therapy sessions. Adding bumps can alter the speed. Flags can be purchased from different types of sport stores to help add an object to go around.
Boots are a source of discomfort for most skiers. Consider lightweight Alpine touring boots for those with coordination difficulties. There is greater range of motion available at the ankle. And it can be turned on and off.
Locating a lesson from a staff member can be challenging. Consider renting a house or finding somewhere off road in a park where you can legally try feet on skis. Expenses can be frustrating. They promote poor behaviour management and add distress.