For you crafty types who don't happen to have an old wooden pet kennel lying around, here are the dimensions of our imagination car. Roughly. I didn't build it, so you're on your own for a pattern, but I would think if you're handy you could figure one out with these photos. I would round up, rather than round down, for all measurements.
Click on the photos to embiggen.
As you can see, the inside is one large space that extends under the seat.
Yes, somehow preschoolers can fit through that hole. Sort of.
I can't advise on the detail here, except to say that the sides are two pieces butted together, that is, there is a straight horizontal seam in the middle. Which means the bottom piece could be the negative space, or the scrap, from cutting the top piece, but trimmed shorter.