Although by invitation is great, I think that kids need to know what is the expectation as a disciple of Christ. Fasting shouldn't just be if you want to or here and there, but with a purpose and kids need to know that. Doing it as a family is great because they know that their parents are doing it too. After enforcing a time or two with a purpose in mind, then maybe having them choose the purpose could lead them to having a spiritual confirmation early in life. If they fight you at it, then there are maybe things you can look up in the Friend to help them see that its not just you pushing it, but it's a commandment but one that yields tremendous blessings. Share your experiences. Then show them pictures of kids in Ethiopia and that surely they could miss two meals even if its just to be grateful for what they have.
