childrens picture book

[Debora]

Need title of childrens picture book. Story was about animals that dressed in disguise to look like children in order to attend a party or a parade. I read this short picture book to my children in the early 1980's. The illustrations were beautiful. The animals I remember:
a bear, maybe a lion--maybe zoo animals?? I think the story begins with "children" attending an event with party hats or balloons and the surprise ending is you see the animals zip out of these "children" costumes and they are animals....something like that. Quite clever story.