So after that happened (yep, I only let that happen once!) my solution was to put together a "car box" filled with all the things needed to make sure that everyone is dry and happy for a peaceful car ride. In my box, I included: diapers, a container of wipes (great for diaper changes or sticky hands), snacks, a couple of juice boxes, a bottle of water or two, and a few small toys and books (ones that the kids weren't used to or had never seen). I used to keep the box on the floor of the backseat so it was there whenever I needed it.
Dry diapers, full tummies, and entertainment in the car makes for a happy mom!
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I never let the kids into the "car box" on their own, but I did have a bin that they could reach their own full of toys and books to keep the kids busy. For me, with two kids, it worked perfectly having a box that fit perfectly in between the two carseats. I stood some books upright at the back and then kept an assortment of "car toys" in it so that the kids always had some things to keep them happy in the car. And I rotated some of the less played with toys frequently so they would regularly have something new to look at. And with having the toys in a box, it made it really easy to put the lid on and stash it in the back when someone needed to sit in the middle seat.
Happy car trips!