I used National Express for many years, 4-5 times a month, between LGW and LHR and whilst they could be good, they could also be very, very bad. If you are linking LGW-T5 there's not to much that can go wrong and it's a straightforward journey. If you are going to Central Area (T1,2 and 3) you've got to add 15-30 minutes for getting into/out of Central Area after the T5 stop. If you are headed for T4 you need to transfer to the previously mentioned shuttle bus.
With four to manage it is better to think of a taxi service.
Agreed - the last time I looked National Express charged the best part £20 STG per person, single fare, for the journey between LGW and LHR. You should be able to get a taxi/car for not much more than that and potentialy have a much quicker and more comfortable journey.