We have an au pair so our childcare expenses are fixed no matter how many kids we have. As for the car situation, we drive a minivan for all the kids, I picked up a cheap "commuter car" to drive to and from work so we could leave the van home fore the au pair. If you don't have the $$ for another car, there are several car seats out there that are narrow enough to fit three in a row in most small cars, Radian and combi come to mind off the top of my head. We have a nissan versa and honda civic for our "commuter cars" but we also use them when we don't have to take all the kids with us, and we can fit several combinations of seating in the back. My oldest is 7, but he is still in a 5pt harness seat. I
f you are going to purchase seats for your nanny's car, just have her come with you so you can "try before you buy" the seats.