MY car was in for warranty work last week, Tuesday morning through to Friday afternoon (for warranty work on paint, rear tailgate) and they gave me a Candy White Bluemotion Golf Convertible for 4 days, in the (very small) boot was the the price tag so they can deffo use demonstrators or possibly used cars (was not sure which that was as it was a 61 plate and had done 7k miles).
Personally I would tell them you need a bigger vehicle as you have family and they are not going to fit in the fox.