What is the full form of children day?

"Children's Day" doesn't have an official full form. It's simply a term used to celebrate children. However, it might be used in conjunction with other terms, like:

* International Children's Day: Celebrated on June 1st in many countries.

* Universal Children's Day: Celebrated on November 20th by the United Nations.

* National Children's Day: Celebrated on different dates in various countries.

It's important to specify which "Children's Day" you're referring to, as the date and meaning can vary depending on the context.