A "nap" is when you sleep for a short time during the day. Young children take naps a lot. Some adults would like to take naps too if they could.

The most common verb to use with "nap" is "take":

I'm going to take a nap.

But "taking" a nap sounds like something that a person does on purpose. When people talk about children, they use the phrase "have a nap":

Has she had a nap yet?

Oh, you're so sleepy! Why don't we have a nap?

This makes the nap sound like something that the child needs to do each day.