I don’t think a kid in a car seat would come anywhere near the airbag zone. Unless maybe the seat broke – which it could – in which case the airbag would be useful I think.
The issue with rear facing seats is that they can come close to the dash, and if the airbag goes off it can smack them in the back of the head, which causes worse injuries than the crash you may be having.
I did see a tv programme a while ago about older kids of 5-12 or so being killed by airbags in the States. They’d be leaning forwards to change the radio station or whatever and with their heads closer to the airbags the shock would kill them. However the conclusion was that it wasn’t an issue in the UK because our airbags aren’t as powerful.
I reckon it’d be okay – not as safe as the rear, but you have to balance that with the kid’s enjoyment and development.