The children laughed merrily.

simple
compound
complex
compound-complex

This sentence is a simple sentence. The answer would be "Simple".
 
A compound sentence would look like this:

"Annabelle raced to the toy store and she begged her mother to buy a toy at the store.

A complex sentence would look like this: 

"If you touch the hot stove, you will get burned."

A compound-complex sentence would look like this:

"While I ran to library, Keith strolled leisurely behind me, and he enjoyed his ice cream cone whereas I tried to gobble mine down."

I hope this helps you.

You need to breathe, and just count to 10 slowly, it .