Hi,
Thanks for the interest in our plugins. I can answer your questions.
What you are describing we call Global Stock and it is going to be a feature in 4.1, which we are working on and hope to release by the end of the month.
It will enable you to set a stock of 30 for an event and then you could sell different ticket types like adult and child and only 30 total tickets would be sold with any combination of the two types or more if you set it up that way.
Let me know if you have any follow up questions.
Thanks