Every Room Type setup allows you to define the Maximum number of Guests and Maximum Number of Adults per Room.
If the Number of Adults is less than the Max Number of Guests, the difference will be = number of children
In this Example, the Room takes a maximum of 4 guests. The maximum number of adults accommodated is 2 and the maximum number of children accommodated will be assumed as 2.
Should you require to charge for children staying at the property, the Rate Plan setup allows you to define the charge for them.
Try making a booking on your Website Booking Page and ensure, it is calculating the way you want it to.