Good writing has a rhythm and flow to it. One way to achieve this flow is to mix up sentence lengths.
For example, the following paragraph has very little flow to it:
"The Titanic was an amazing ship. It had lots of fancy rooms. It had a gymnasium. It had a dining area. There was a huge chandelier. It even had a pool. Titanic was the first ship to have a pool. First class was on top. Third class was on the bottom."
The paragraph above is awkward because most of the sentences are short, and many of them start with the same word. Many fourth graders begin the year writing like this. Now consider this paragraph:
"The Titanic was an amazing ship. It had lots of fancy rooms, a gymnasium, and a dining room. There was a huge chandelier. It even had a pool, and Titanic was the first ship to have one of those on board. First class was on the top, and third class was on the bottom."
There is a mix of long and short sentences, improving the rhythm and flow. Few sentences start in the same way.
To help your child construct stronger, more varied sentences, help him or her revise constantly. Students should always read over their writing several times, and one of the things they should be checking is their sentence fluency.