Moira’s Birthday by Robert Munsch, 1987.

Moira wants to have a birthday party and to invite every kid in her school, from kindergarten to sixth grade. Her parents say that she can have only six party guests total, not six grades’ worth. However, so many kids at school want to come to her party that she ends up inviting the whole school anyway.

When every kid in school shows up on the day of the party, Moira’s parents are bowled over. There are so many kids that they hardly fit in the house. Moira calls up a pizza place, asking for an enormous amount of pizza, and a bakery, asking for an enormous amount of birthday cakes. The kids also help out by supplying their own food from home.

Naturally, Moira’s house is a mess, and her parents are upset, but Moira, seeing the enormous pile of birthday presents that everyone brought, promises a present to everyone who helps to clean up.

But, just when everything seems to have worked out all right, the trucks from the bakery and pizza place show up with the rest of Moira’s order.

The book is currently available online through Internet Archive.