There are expenses that you know will end at a time in the future. These include childcare, and also expenses that will end when a child leaves the home or becomes emancipated.
Enter the expense on the Living Expenses page (Client Info > Income & Expenses > Living Expenses).
Click more info on that line.
Enter an end year.
For a final partial year, you may override the expense in the final year, at the bottom of the page.