We bred our lil' vegans that way from the start!
Second: how old are your kids? This all makes big differences.
My kids eat vegan because that's what we have. Rarely, but rarely, the slip up at friends houses or with relatives. A bit more than my wife and I, since we don't trust our relatives nor our kid's friend's parents, but our kids do.
Each of our kids eats differently, so their approach to vegan food is different. My daughter eats greens and veggies without abandon, and she eats plenty. Her older brother likes processed foods and candy and doesn't like eating except for drinks. Obviously, we have to address them each differently.
What do your kids like? What have you tried? How old are your kids? Do they know why you went vegan?