It's nice to hear about your family's experience, particularly your son's birthday celebration at Fort San Pedro. The free entrance for kids on special occasions is a kind gesture by the staff/management.
The structure of Fort San Pedro looks similar to Fort Santiago we have here in Manila. I hope I can also visit that place, too!
Yes they are kind and accomodating kuddos to the management to hire those staff that accomodate their guest and give discount especially for kids, hopefully if you can travel here in Cebu I can show you the place.