Depending on the time of your Family Formation session, you might try
serving a meal (or having a potluck) before or after your session.
While the 5th graders are taking the test, you could have a social time
for the families, or perhaps have them work on preparing some of the
Lenten activities in their packet. This way, no one would have to make
a separate trip for the test, just come a little earlier or stay later.
Plus, you have the additional bribe of some food and social time for
your families. You might even have families without fifth graders who
would like to come.