In my '04 X3, when no one is sitting in the passenger seat, the airbag is deactivated. (as shown by the warning light in the overhead console). It may be this way even with a baby seat.

If you wish to disconnect the passenger dashboard airbag, the connector is on top of the fuse panel, which is located at the top of the glove box. To get to the airbag in the A pillar, the connector will require popping off the A-pillar cover. And then there is the door airbag - not sure where that connector is located. One part of the X3 I haven't had to take apart...yet.

Good luck.