Extra fee for children without changing max occupancy

I am trying to set an extra fee/extra guest, but it seems that children are not included. So although the apartment is at max occupancy 5, only 2 adults will pay because the rest of 3 guests are children. 

I tried to change the policy on children fees but I can't do it without changing the max capacity which would turn into 9?! 

How would you advise to go about it?