布小熊

I don't think 2 car seats will fit at the back seats of a mini (or IS250).

布小熊

All children over nine kg (40 lbs) need to be in booster seats until they are 1.45 metres (4’9”) tall or age nine.

http://www2.news.gov.bc.ca/news_ ... PSSG0040-001010.htm

Assuming one girl is 9 already, they still need to try fitting one booster in the back seat at least for another 3 years.