The law does not prevent you putting the child in the front of a vehicle fitted with an Airbag. The 'very strong' recommendations are never put a rear facing seat in a seat with airbags. How old is you grand child? If they are not in a rear facing seat then you can use the front seat. If the child is in a rear facing seat, is it worth the effort for the short time before they are in a booster?
You may find this FACT SHEET
and Is also useful
Before deciding that the risk of airbags is too high, I suggest you research the facts rather than rely on the generic advice that presumes a back seat with air bag is readily available. The only thing you can reliably state is "Rear seat is safer, use it if you have one" ( which begs the question, why we allow our partners to travel in the front seat beside us.)
There is a lot of scaremongering around this subject, and while not PC to call it out when discussing the safety of children, I am prepared to call it out - the research I found with my children was old then - well over a decade ago now, and the additional risk, while statistically significant, largely not quantifiable because the numbers that were poorly restrained and not in booster seats.
The problem now is all cars have front airbags, so researchers have no base line to compare to results to, and airbags have changed since the early to mid 1990's when the first research was done, so its not possible to apply those findings to modern vehicles.
Don't be alarmed when you see me talking to myself, I am getting expert advice.