Not sure specifically about your car, but:
1. In the olden days this was an optional feature called 'child seat recognition system', that if fitted would automatically disable the front passanger airbag when a Mercedes Benz approved ISOFIX child seat is detected.
2. On all MB cars (whether fitted with the optional 'child seat recognition system', or not) there is usually a dashboard light for 'Passanger Airbag off', that lights-up briefly when the ignition is turned on. If your car has this light, then that would be a good way of knowing if the front passanger seat has indeed been disabled (i.e. the light should come on permanently when the child seat is connected).
3. There's a separate feature on all MB cars called 'front passanger seat occupancy sensor' which disables the front passanger airbag when the passanger seat is unoccupied. This works differently to the 'child seat recognition system', and it's operation is seamless in the background i.e. there's no dash light or indication to show if it's on or off. The point is that you can't relay on the child's low weight deactivating the airbag, you need specific indication that the front passanger airbag is indeed decativated, as per (2) above.
4. On a general note, it is not recommended for children under 12 years only (some say 14) to seat in the front passanger seat. If you must sit a child there (with or without child seat), then you should move the front passanger seat as far back as it will go, and recline the backrest away from the dashboard while keeping it flat with the rear of the child seat.
