You should always put an older kid in diapers for road trips.
Diapers on road trips means the kids will enjoy the road trip more and you won't have to stop for potty breaks and the kids will be more comfortable to use their diapers instead of holding in the pee.
All kids even potty trained should wear diapers when on a road trip because it means they don't have to hold their pee in.
Wearing a diaper on a road trip even when potty trained saves the pain of having to hold the pee in while you look for or wait for a rest stop.
My kids wear diapers on road trips and I let them know it's okay to just use the diapers.
Then when the diapers get full and we're at a rest stop I change their diaper.
My kids wear Luvs size 6 diapers on road trips and they hold up good.
Sometimes they can go a few hours or so between diaper changes and they never even use the toilet on the road trips because the diapers are more convenient.
An 8 year old boy should be able to wear Luvs size 6 diapers.
Luvs size 6 diapers have pretty stretchy tabs that stretch quite a bit to fit.
Pampers baby dry size 6 may also fit the 8 year old boy or even size 7 pampers cruisers diapers or size 7 Gentle steps diapers.
If your 8 year old boy won't fit into the size 6 Luvs or other size toddler diapers then you can buy a Youth sized tape on disposable diaper online such as Attends Youth diapers.
A 7 year old boy can wear Luvs size 6 diapers if they are skinny enough.
If the 7 year old boy is an average size and skinny built then they should be able to easily fit into a size 6 Luvs diaper.
Luvs diapers have really stretchy tabs and they should easily fit a 7 year old boy.
If not then a size 7 Pampers Cruisers or Size 7 gentle steps diaper should easily fit the 7 year old boy.
A 5 year old boy can wear Luvs size 6 diapers for bed wetting.
The Luvs size 6 diapers have really stretchy tabs so they can easily fit most kids of 5 to 10 years old just fine.
A 5 year old can wear Luvs size 6 diapers.
If the 5 year old is skinny enough and average weight and not overweight then the Luvs size 6 diaper should fit the 5 year old boy just fine.
I have a 5 year old boy that wears Luvs size 6 diapers because he's not potty trained yet.
He wears the Luvs size 6 diapers day and night and they fit him just fine.
His 7 year old brother also wears Luvs size 6 diapers as well for bed wetting and when away from home.
The Luvs size 6 diapers fir the 5 year old boy and the 7 year old boy just fine.
I'm sure they'll both still fit in the Luvs size 6 diapers even when they are 8 and 10 years old because they are really built skinny.
However if your kid is bigger then they would need a size 7 diaper or a Youth diaper.
But for most skinny sized kids a Luvs size 6 or even size 7 diaper should easily fit them.