My 6 year-old likes this:

https://www.amazon.com/gp/product/B018FX1JNA/ref=oh_aui_detailpage_o06_s00?ie=UTF8&psc=1

My kids (9,6,3) are all in car seats that still have high backs and therefore somewhere to rest their heads. The younger two can have their seat reclined a little. We have a mini van which helps with space. They often choose to take the little top blankets from their beds to snuggle with which also helps.

are they complaining about not being comfortable and thus unable to sleep or do they just look uncomfortable? My kids often look uncomfortable but they sleep just fine in the car. My older one is in a high back booster which is nice because it gives him a headrest to lean against.

Our kids take along the pillows from their bedroom and scrunch it up against 5he window to rest their heads. Travel pillows never worked for us.

Maybe consider changing things up. We road trip a lot in my sedan, but I travel when kids are up and I'm more alert. They take light naps occasionally (now 7 and 8, but started when they were 2 and 3). it is so hard to have them sleep in the car when kids tend to flop down (rather than sit upright) when they sleep in the car. Perhaps a dog bone pillow would work for them.

We do a lot of road trips and like to travel in evening so kids sleep in car.

But they get so uncomfortable, whether in car seat or booster or just seat belt. We tried neck pillow but no help.

We have a sedan so maybe folks are more comfortable in like minivan captain chairs? What do other people do, how to make backseat comfy for kids to sleep??