What we did with the "nature's call" issue, when our kids were growing up, is call out when we'd see a sign for a rest area, "Does anyone need to go?" We would stop if one of us needed to go, but EVERYONE would visit the facilities. We'd let them know, if you don't try now, don't come crying to us in 10 minutes that you need to go!
Each girl packed her own bag of small toys and games that could be played with in the car. It stayed with her. They also had Cheerios and other small finger snack foods -- yes, they got all over the car, but it cleans. A sweater or jacket was kept nearby if we were traveling at higher elevation or in a cooler climate, otherwise those stayed with the suitcases. We had to "make do" for cupholders, but we always tried to get travel cups with lids.
As the navigator and overnight-finder, the front passenger area had a bag of maps, hotel/RV park books (depending on what we were traveling with), and "The Interstate Exit Authority" (now known as "The Next Exit"). That very handy guide tells us what's at every interstate exit and where the rest areas are, so for that plaintive cry "I gotta go!" I can search for the next truck stop or rest area.
After a few days of travel, you'll get your "travel rhythm" and have your vehicle set up just the way you like it. My husband and I know where each thing goes in the cab of our pickup, after almost 10 years of road trips. We can practically pack it in our sleep.