Various Ticket types, all coming from the same inventory

Hi all, I'm new here and am hoping to help out my dad who sells tickets to tours/events he runs. He runs the tours on specific days, and has a limit of 25 people. The complication is that adults cost more than children (he feeds them too), and so he needs to charge more for the adults.

So, it's a "Service Product", with a few variations (Adult vs. Child), and each purchase needs to come out of the same inventory (25 in total). Is there a way to do this out-of-the-box with SquareSpace, or must I use some plugin type thing that will cost him more?

Many thanks in advance for any advice you can offer....