This takes some preparation before leaving home but is something your kids can put together on the way.
I make a business card on the computer with a phrase like "Remembering Our Relatives" and adding my name, address, phone number and e-mail. Then I get a pack of plastic luggage ID card holders that can be laminated. They already have a hole in them. You can also use business card size holders. I put the cards in the plastic and laminate them. If you get the plastic card holders without the hole you will need a hole punch to put a hole in the plastic.
Then I get several bunches of artificial flowers. The ones which have removable flowers. I remove the flower and slip one of the laminated cards on the stem and then replace the flower. When I find the grave of someone I think is related to me I put one of the articial flowers in the ground by the tombstone and hope that someone into genealogy will come along, see my card and call, write or e-mail me. If they don't allow artificial flowers, use one of the small flowers like a petunia from a six pack.