Disable "Submit" button if nodereference field is empty

Parent logs in and creates "children" (nodes)
Parent clicks on "Register for Baseball" and a table with divisions and age ranges comes up. Parents are to locate the division their child is eligible for on the grid and click the link to bring them to the registration page (paid event). On that product page I have added a nodereference using a view to display all of the children associated to the logged in user whose birthday falls within a predetermined range, so only the children that are eligible for that Division are displayed.

In theory, it works great... until the parent clicks on the wrong Division and NO children are displayed (and the field disappears entirely). There is simply a message on the top of the product page which reads, "The webform component nodereference is not able to be displayed." and the Submit button is still activated which causes them to complete a registration for an unknown child.

When there are no children in the eligibility age range, I need a message to come up that reads, "you have selected a Division for which your child is not eligible. Please return to the registration page and select the correct Division." and I need the submit button to be disabled.

Thank you!