As with all of the VV games, make sure you have your children picking the mushrooms that appear anywhere on the island.

You can get general food tips in this thread.

Otherwise, you can take a look at the official VV3 Guide which contains major spoilers on how to feed your villagers or you can take a look at this thread. Keep in mind, both of these contain major spoilers about the game in general so if you don't want anything wrecked for you, I would suggest taking a look at the first thread I gave you.

Hope this helps!