What is the name of poem spoken in Christmas Carol with forty children conducting themselves like one?

The poem you're thinking of is "The Christmas Carol" itself, which is spoken by the Ghost of Christmas Present.

It's not a separate poem within the story but a part of the Ghost's speech:

"And they know me. Know me? Yes, boy! They are all his children. They are all his children!"

The Ghost then goes on to describe a scene of forty children, all playing together in a harmonious and united way, acting as one. This is a key moment in the story as it emphasizes the power of unity, love, and compassion, all essential themes of Christmas.