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.