Anybody can wear diapers when they need to or want too and even older kids.
If your kid wets the bed at night and falls asleep on the long car trip then they should wear diapers for road trips to keep their pants dry and your seat dry in the car.
I had my kids wear diapers on long car trips when they were 6, 7, 8 and even 12 years old when they were wetting the bed.
My kids once had an accident and peed in their pants on a car trip during the day and it took some cleaning to get the pee and pee smell out of the car.
So after that I had them wear diapers when going on long car trips where we couldn't stop often enough to use the restroom.
They were reluctant at first but I explained to them that they would be much more comfortable wearing diapers than wearing wet pants or having to hold their pee in until we found a restroom.
I let them know that if they needed to pee and we were not near a restroom then they could just pee in the diaper and change it when we stopped or when it got full enough.
They wore pampers size 7 diapers when they could fit in them at age 6, 7 and 9 years old and then I bought them Youth disposable diapers online which they wore.