Your piggies were not able to reach the tree to eat because of the piggy stick. They were starving -- I could tell because I could not nibble them, they said meat out, dude. Move your piggy stick closer to the tree, or just remove it entirely. You don't really need it. They should probably have access to more trees anyway. Alternately buy a piggy feeder. For safekeeping I used a Potion of Animal Youth on them and gave them to your butler. Just feed them to make them into adult piggies again.