Airbags shouldn't deploy from the rear impact. Front airbags should deploy from front hit, and side airbags will deploy from a side impact (on the side of impact).
I think a malfunctioning airbag will trigger a light on the dash, but you can ask a dealership to run diagnostics if you are worried.
The rear seat is considered to be the safest place for child seats.
2008 E90 335i 6MT + SSK + CDV-delete / ZSP / aluminum / black leather / Conti DW + compact spare / integrated portable nav plus
2005 E83 X3 Auto