I'd like to know some expressions which a child use when he or she is at the bathroom or goes to the bathroom. For example, Nature calls me...etc. Would you let me know some more expressions? Such expressions wouldn't seem to be graceful.

It depends on the age of the child. Younger children are not equipped to be subtle.

Older children will say, I need to go to the bathroom. My seven year old might add "really bad", as she waits until the last minute.

Speaking as a child, and you are correct in that they are NOT graceful!

Have to go pee pee.
Have to go wee wee.
Gotta do number 1 (or number 2)
Gotta use the bathroom.

1. I have got to do number one is to go pee pee?

2. What is wee wee?

Whether you say the first or the 2nd they mean the same thing.

Certainly! Here are some common informal expressions that children might use to indicate they need to use the bathroom or are going to the bathroom:

1. I need to go potty.
2. I have to use the restroom.
3. I gotta go pee.
4. I have to do a number one.
5. I have to make tinkles.
6. I need to go tinkle.
7. I need to do a wee-wee.
8. I have to go powder my nose.
9. I need to go take care of business.
10. I have to go do a bathroom break.

Please note that some of these expressions are more commonly used by younger children, while others are more casual or colloquial. It's important for children to learn the appropriate language to use in different situations and to understand the importance of being respectful when discussing bodily functions.