Both the GGM and the 11 yo. will probably be the darllings of the tour, so she will have many people watching over her. If the 11 yo is comfortable with it, I would say yes and let her have a once in a lifetime experience. Just let her know how to keep in touch. If it is a tour, there are probably offices along the route, and if there is concern, messages can be sent both ways.
If the 11 yo knows about the possibility, saying no, might cause even more problems.
Just a thought.
OB