Hi, 

I am finding that reservations with children (eg 4 covers comprising 2 adults and 2 children) are consistently finishing earlier & spending less money than a similar booking of all adults. 

This makes sense really. Children can only sit still so long, so an hour and a half is ample. The equivalent table of all adults will linger, have more drinks and usually pudding & coffee. 

Is there a way to apply leave time rules to reflect this reality?

I can think of ways to create individual promotions however this would exponentially grow the number of promotions we have.