With two children now (aged 2½ and 11) and a propensity for having picnics wherever we go, essential equipment in our car includes least-cost materials:
1 lu roll in the car.
1 lu roll in the boot (trunk) as a backup supply.
1 kitchen (tissue) roll in the car.
1 kitchen (tissue) roll in the boot (trunk) as a backup supply.
2 plastic shopping bags in the car.
1 plastic bucket in the boot (trunk) as a standby.
1 compact steel-and-wood utility shovel in the boot (trunk) for when needed.
It's surprising how these things can make life so much simpler and stress-free. The lu rolls and kitchen rolls are great for cleaning-up snotty noses, and cleaning messy hands/faces after picnics in and outside the car, and for necessary roadside toilet-stops. The plastic bags are for urgent/unexpected vomits, and the bucket is for if anyone is looking like they might
be about to vomit - you can't always stop the car safely in sufficient time to let them get out of the car to do it. The shovel is for burying the waste evidence with earth, so that it is tidier and decomposes faster.
So there's never any need for panic or trauma when a natural bodily function urgently kicks in.
Famous quote from our daughter after her first urgent roadside stop:
"I can't believe I did such a good poo on the grass by the roadside!"
The bonus with the bucket and spade is that they are dual-purpose - most often, they are used to make sandcastles when we stop at a beach!