I have some professional knowledge on this subject, albeit somewhat out of date as I've not worked on child restraints or airbags for a good 10 years....
Passenger airbags in the front aren't strictly necessary - they came about through marketing and demand for 'equal' protection to the driver (ignoring the fact that the driver has a steering wheel in front of them that will do them serious harm if you interact with it in an accident!). We've since moved on in terms of protection and for an adult they do offer a potential benefit if the accident is centred on the passenger side of the car. Side airbags are a significant benefit.
Posts above are correct that with any kind of child restraint in the front, switching it off is a good idea, as the child restraint will inevitably sit nearer the airbag than an adult occupying the same seat. It absolutely must be off if the child restraint is rear facing, as an airbag going off in those circumstances is very likely to do serious harm.
I'm in a similar position to John, in that my 10 year old has outgrown all classes of child restraint - in our S5 it got to the point he was sitting in it in the back with his head kinked over to one side as it was making his head hit the roof of the car!
He recently went down to the West Country with my wife to visit his granny in our Golf. He sat in the front so he could see better and deal with music selection for my wife. I switched off the airbag, because the one thing I can't guarantee he wouldn't be doing in the event of an accident is leaning forward to pick something up off the floor, or reaching into the glovebox. Assuming he's not doing those things, the airbag won't actually help anyway, but if he is it's quite likely to do him harm.
Risk management is a very personal thing, so I would never criticise someone for making a different choice, based on their own circumstances and knowing their own child
Adrian