I used to get asked the same question when I worked for Ford.
It is possible to retro-fit a switch (as per those fitted in production), however, you may find it difficult to engage the services of a dealer who wishes to do so. As it carries a high degree of liability, in the event of a crash. (Disclaimers may need to be signed, and insurers will certainly need informing)
Although, this cost & hassle may well be unnecessary in any case, as a father of 2 kids (the younger being 1.5years), as I've interpreted the law/consumer advice, its only when the child is sitting in a rearward-facing child seat, in the front of the vehicle, that the passenger airbag must be disengaged. If the child is facing forward, then there is no problem, although every effort should be made to have the seat slid furthest away from the dashboard (which presumably would not compromise the rear occupant).
Hope this helps to clarify your options.
Thanks for that. I was thinking along those lines to be honest. Smart say the same about their passenger airbag system - move the seat all the way back. Probably assisted by the fact they haven't got a back seat to use and don't want to lose any potential customers with children....