I also want to thank my friend Howard for sending me three very helpful links regarding tombstone rubbings.
There is much good info here: http://www.gravestonestudies.org/faq.htm
Special problems in Connecticut: http://www.ctgravestones.com/Conservation/Rubbings.htm
There may be other similar things in >other states: http://www.gravestonestudies.org/resources.htm
Because tombstone rubbings are so controversial and may not be allowed, I came up with another idea of an art-genealogy activity with my grandchildren. I realize not everyone will have access to what is required for this project, but lucky me, I'm married to a man who collects the items needed!
Coffin plates. These are the decorative metal plates, often beautifully engraved with designs, which have the deceased's name and dates of birth and death. These metal plates were never meant to be attached to the coffin. They were usually propped on a stand during the funeral then given to the family of the departed person, as a memento. 
In earlier years some were attached to the coffin and buried but around the 1860s it began to be popular to keep them in the family to remember the loved one who had died. Different countries have different traditions of course and not all countries used coffin plates or if they did, they did not consider them a family keepsake. 
In any case, we have a fairly large collection of these beautifully decorated coffin plates. My oldest grandson has always loved sitting on the steps with his grandfather, reading the ornate script and asking questions about the individual whose coffin plate he held. Every summer they follow the same routine - out come the coffin plates that are not on display and our grandson spends a happy hour or two with Grandpa. 
Some of our coffin plates are of ancestors long deceased (such as this one for John Elgie, a great uncle), but most are not family members. We do however research the names on every coffin plate we obtain, so usually we have a pretty good family history on each one. I think it will be fun and interesting to talk about the one(s) they choose - the time period it was made, the artwork and the person for whom it was made. We will let each of the grandchildren choose a coffin plate (or plates) that they would like to rub and have framed for their wall. I think they are going to enjoy this activity, and I may even save it for a rainy day!
The first thing to do is to gently and carefully clean the tombstone you have chosen. Don't use harsh cleaning materials or soap, just a bit of water and your soft brush. You want to remove any bird droppings, moss, etc if possible. But be careful to not damage the stone in any way.
Take your rubbing wax or crayon and start rubbing around the edges of the tombstone. Start lightly, as you can always go back and darken the rubbing later. When the edges are done, move to the central part of the stone. It's a personal choice as to how dark or how light you want the rubbing to be.