I thought about that aspect, the approach I would take is not to call the event a "Father/Daughter Dinner", but a "Sweetheart Dinner" (since it's around Valentines Day) and invite the girls of our school to attend with the "special man in their life (father, grandpa, uncle, brother, family friend, whoever their male role model might be) and the same for the event for the boys. Another school in our town has done this events for the girls of the school for a few years and they always have a good turnout and lots of support.
My only concern would be what about those daughters and sons who don't have fathers and mothers available to attend these events? I think they would feel really left out. Maybe something to keep in mind when planning something like this. I don't mean to say it's not a good idea, but at our school, there are lots of kids without both parents at home and some with one parent completely out of the picture, so this would not be a good event for us to promote. Just my 2 cents.
What about a mother/son sports night-- where you have all different games set-up to lay-- volleyball/ basketball/ maybe some spots with game systems with games that mom/son can play together--- also a dance with a sports theme--
Or a night at the movies for mom/son-- talk to a movie theater about renting 1 theater and pick something appropriate for everyone to watch...
We do "Muffins with Moms" but that might be hard to restrict to just sons. You could do a dinner where the boys are there to serve their mom's...we used to do a PB&J Cafe so you could run one where the boys invited their mom's to Lunch and served them at their tables.