You have 2 options to deal with this situation.
1. Allow the guest request, 1 seat goes empty. 1 person has to wait for the next train. The family gets the souvenir photo that last forever. Everybody is happy.
2. You hold up the line, anger the family, anger everybody else in line wondering why several trains are being loaded, but not this one, interrupt the manager CM to come over, and create a several minute delay, the family remembers that moment forever, all because we want to stick it to somebody.
This isn't bad guest behavior. Taking flash photos is, line cutting is, bumping into me to make the line move faster is, inappropriate conversation in line is, not watching your children is, and I can go on and on. Asking for special seat assignment is not bad behavior.
So, how is option 2 better again other than being authoritative?