I am thinking of having a 3rd child but just got a kia soul 2 months ago and was wondering if 3 would fit in the back seat? I have a 7 year old, a 4 year old and then would have a newborn seat in the middle. I'm thinking the 4 yr old would still be in a high back booster and the 7 year old wouldnt' need a booster anymore so what do you think?