In reply to gymwear:
Personally I would pay for the onsite parking and get the bus. Whilst many of these companies are legit, many are just dumping cars by the road side and on scrap ground around heathrow not to mention reports of people getting speeding notices for when they were away.
Much rather park it myself. Why not drop kids, wife and luggage yourself, then go park and get the bus back... your wife can start queueing for check in and shouldn't take much longer overall