Can I set a different price for children?
Yes. A trip can carry a separate child rate, and guests choose adults and children when they book.
A child rate is worth setting if you take families, because it makes your listing noticeably better value to exactly the guests most likely to book an inshore morning.
If you have a minimum age for a particular trip, say so in the description. It saves an awkward conversation at the dock.
Set the child rate on the trip and say in the description what counts as a child — under twelve is the most common line here, but it is yours to draw. Guests choose adults and children separately when they request, so the price they see and the deposit they pay reflect your rates without any arithmetic at the dock. Life jackets sized for the children on board are your responsibility, and a family that told you the ages in advance will expect them.