If there is no charge for child then child price can be set to 0

for example:

1 Child=0
2 Child=0
3 Child=0
4 Child=0
 

case II:

Lets say if , up to 2 child are allowed . then after extra child is 50$ then you can do like :

1 Child=0
2 Child=0
3 Child=50
4 Child=50

If you want to set more than this then you can always use extra field parameter.