I'm a Frog by Mo Willems
Gerald is careful. Piggie is not.
Piggie cannot help smiling. Gerald can.
Gerald worries so that Piggie does not have to.
Gerald and Piggie are best friends.
In I'm a Frog! Piggie has some ribbiting news! Can Gerald make the leap required to accept Piggie's new identity?
Pretending is not a new concept in the Piggie & Elephant series and I don't think this particular book was nearly as funny as some of the others, but it is the usual Piggie and Gerald shennanigins that I know children love. And as a wink to the adults (for there always is one) there is the following dialogue:
"Even grown-up people?"
"All the time."