To get the option to select grandchild, follow the steps below:

1. Click on "Personal Info"
2. click edit next to the Dependent (if you already stated or "add a dependent" if you haven't started)
3. On the screen that asks "Who else do you support", select the radio button next to "Another person (includes half and steprelatives and in-laws)"this will open up a drop down, in that drop down you will be able to select "grandchild (see image below).