Open the resource settings.
Specify the total share count — the total number of adults and children's tickets.
In our case it is 20.
Then create 2 shared price options: Children and Adult.
Now in the children's price option, specify the price cheaper than in the adult.
If both 20 adults and 20 children's tickets are available for booking, then in both price options set Share count to 20.
In result, you will get this configuration:
You can also strictly divide the total number of tickets between adults and children.
For example, allow booking up to 5 children and 15 adult tickets.
To do this, specify Share count in the children price option — 5.
In the adult price option — 15.
In this case, the configuration will be like this:
See this example in action: