Hmmm.
1 Adult and 1 Child to LAX gives you this:
All Passengers
Taxes, fees, charges & surcharges description Code Amount
Operating Airline Fuel & Insurance Surcharge YQ GBP 478.00
USA Animal & Plant Health Inspection Service Fee XA GBP 6.40
USA Immigration Service Fee XY GBP 8.80
USA Customs User Fee YC GBP 7.00
UK Air Passenger Duty GB GBP 130.00
UK Passenger Service Charge UB GBP 68.98
September 11th Security Fee AY GBP 3.20
Total GBP 702.38
1 Adult is:
All Passengers
Taxes, fees, charges & surcharges description Code Amount
Operating Airline Fuel & Insurance Surcharge YQ GBP 239.00
USA Animal & Plant Health Inspection Service Fee XA GBP 3.20
USA Immigration Service Fee XY GBP 4.40
USA Customs User Fee YC GBP 3.50
UK Air Passenger Duty GB GBP 65.00
UK Passenger Service Charge UB GBP 34.49
September 11th Security Fee AY GBP 1.60
Total GBP 351.19
So yup - looks like they all apply...

Now, that's for a 2-11 child. Not sure about an infant.