We are planning to do the same. My parents (still living) have theirs on the plots already. They also blessed us with the gift of pre-planning their funeral. That takes care of everything, even transporting to the cemetery, which is in another state. I hope to be able to do that for our children, but will need to wait until our youngest is out of college and independent of us.
Another thing our family has started doing is including on the back of the stone 'Proud Parents of...' and including all the children's names. This will aid in geneology research at some later time.