Either in the glovebox area, or on the left end of the dash, only visible with the door open, will be a switch to disable the passenger airbag. This is operated with the key, and allows fitment of a rear-facing child seat in the front passenger seat. When disabled, that button will light to remind you.
The passenger airbag inflates to about the size of a large pillow, in about three-tenths of a second. So it is fully inflated before you hit it, as the seat belt will stretch to absorb the shock loading, and the airbag is waiting for you when you get there. If there was a child seat there, it hits it with quite a force, and could seriously injure a child.
Instead, wrap the child in a duvet, and lock it in the boot.
Second thoughts. If it is actually a switch, press and hold to disable the airbag. Might be a time limit, so has to be done as soon as ign switched on.
Handbook will give the answer, always worth a try.