I use an external charger for my phone. One of its big advantages is that I can use it to charge my phone without making my phone hard to use (it's about the same size as the phone and they stack together nicely) and then charge the charger without having to attach my phone to the car or wall or computer or whatever I'm charging the charger from. As HaLaBi mentioned in a comment, typically external chargers can charge your device multiple times (or can charge multiple devices if you're a sharer) from a fully-charged charger.
That said, if you are in a car this is just not an issue. Your tablet should easily recharge fully in an hour or less on a typical car charger. I can't imagine there would be more than 4 tablets in the car. So even if all of you were all using your devices full time (unlikely) the charger socket would sit empty about half the time. If you are charging and you get to half full and someone panics in the back seat that they're about out of power, unplug and run on battery for a while to let them charge, then finish your charging. I've handled this long ago with two laptops that had only about 2 hours battery when used to play media from a spinning CD or DVD (that chews battery) and we had no crises, so I am sure that modern tablets playing media from SSD and not pulling down a lot of data over the SIM card will last more than long enough for a single car charger to look after you all.