It should fit her two younger kids seats on each side and Pearl in the middle in a high back booster (a slim one like a Brittax Adventurer would be perfect). I've seen that car take 3 Extended Rearward Facing seats in the back before.She just mentioned in comments that it
Doesn’t fit 3 car seats in the back , so what happens now? They have to use two cars to go places with them all? Or they just ditch a kid?
I had a look in the comments and she said not all seats will fit but hers does. I'd imagine she'll either have a new seat for Pearl or manage to squeeze hers in between the other two.
Also if needed she could fit baby in the front with air bag off and other 2 in the back.