consistent across most places. Target, Amazon, Walmart, etc...

You can increase battery life by reducing the screen brightness, turning off wi-fi, etc like you would a phone or tablet.

4 player platformers:
Kirby is great for that age range. It is easy to play and fun. It allows up to 4 players.
New Super Mario Deluxe would also be good.
My Kids also like Donkey Kong Tropical Freeze on Wii U. The Switch added Funky Kong as an "Easy" mode type player.
Yoshi's Crafted World looks good for that age too.

Mario Party and Mario Kart can be fun, but might be a bit difficult for the 5 year old. My kids didn't really start getting OK at Mario Kart until they were around 7 or 8. Before that it was lots of hitting walls, going the wrong way and being frustrating to play it with.

My 8 year old is really enjoying Legend of Zelda Breath of the Wild, but that is only one player.
You can also get minecraft. Kids LOVE that one. It has multiplayer. Not sure how good it would be on the small screen in the car though. (I don't get it. I love video games but for some reason can't stand minecraft. I hate watching them play it. I have no interest in it. I would rather watch 2 hours of Fortnite videos than 5 minutes of Minecraft.)

If you get a switch and SD card you can download the games you want, so then you don't have to worry about switching game cards in the car, the kids can just press home and switch games that way.