The Trix bunny is a covert operative trying to corner the cereal market by swiping stuff. The kids are wise to this. They put him on the cereal box as a sort of mockery.

As for the leprechaun, he doesn't want to turn his magic against the children. One day though, he'll snap. They'll be chasing him and he'll flip around with his milk straw and blow spitballs at them. Then he'll be tried, sentenced to a loony bin, and spend the rest of his days murmering "they're always after me Lucky Charms...".