Airbag for child-restraint system

An assembly comprising:

• a vehicle seat having a seat bottom and a seatback supported by the seat bottom, the seat bottom having a seat-forward end and a seat-rearward end;
• the seat bottom defining an occupant-seating area;
• an airbag supported on the seat bottom at the seat-forward end, the airbag being inflatable from an uninflated position to an inflated position;
• the airbag in the inflated position having a cavity configured to receive a removable child-restraint system on the occupant-seating area;
• a pyrotechnic retractor supported by the vehicle seat and operatively coupled to the airbag to position the airbag with the cavity at the occupant-seating area;
• and
• a computer including a processor and a memory storing instructions executable by the processor to:
• detect a removable child-restraint system on the occupant-seating area;
• and
• based on detection of a removable child-restraint system on the occupant-seating area, inflate the airbag to the inflated position in response to certain vehicle impacts.