Have a vegetable race like Peter Rabbit. Hide plastic or real vegetables in your backyard, and supply each kid with a basket. Designate an adult as Mr. MacGregor, and have the kids race into the yard and try take the veggies without being caught. If they are tagged by Mr. MacGregor, they must drop their veggie, run back out, and start again. The kid with the most veggies in his basket wins!
Help Jemima Puddle Duck find and keep her eggs! Have an Easter egg hunt! Make it color-coordinated. If the kid has a pink egg in his basket, he must only find pink eggs during the hunt.
Read The Tailor of Gloucester aloud to the kids. Just like in the story, have items laid out—some from the story (such as a mouse, red thread, a coat, a teacup, etc.) and some not. Have the kids use their memories to determine which items were involved in the story.
From The Tale of the Pie and the Patty Pan, have an Easter tea with sandwiches cut into little animal shapes, scones with jam, mini pastries, and chocolate eggs. Assign the kids to sit at the table as one of the characters from the books. They can either decide who they want to be, or you can decide and place them at the table accordingly.
A great idea for a favor is to purchase small Beatrix Potter books to send home with each of the party guests. Everyone appreciates these books: they are classic!