1- Continue to instill in your children the importance for fasting.
2- It sounds like they like many fasting foods already. They seem to understand that they have some choices. It is fine to have different preferences for different foods, especially while they are still young and need nourishment.
3- Hopefully, your children will continue to observe fasts as they grow up. There is no reason to speculate that they will not.
4- It is better to have the children observe fasts normally as we do, but appropriately for their age. Just giving things up is merely an explanation and a visual, but lacks in capturing the meaning for older children. At this age, your children can understand more, especially the 9 year old.