Is it possible to count the total subform rows as each is being added in the record that is being edited?

So in the below example, the "Number of Children booked in" field would be 2. If I added another row it would update to 3, etc. It needs to NOT count any subform records that have already been entered previously.

This field will be used to calculate a fee that will be charged on submission of this parent form dependant on how many children are added.