I can understand that but Weddings and Funerals are unique events. Crown Prince Frederik will only marry once (we hope!) - she can open a fountain or unveil a statue at any time of the year. I would argue that her schedule isn't as hectic as any other sovereign, but for her age, she does do alot. I think it does matter if she goes abroad to a family wedding etc.
I think that by sending Charles and Edward, Andrew or Philip, she is basically sending a message of, "I'm not going - send someone else". Charles can do his own PR, and there's nothing to say he couldn't go as well. When Felipe and Letizia married, Beatrix took Willem-Alexandra and Maxima with her , Carl Gustav and Silvia took all three children. Why can't Elizabeth take Charles and Camilla or Edward and Sophie. It would present a family front. I imagine that the Royals must think, "We'll only get Charles - Lillibet won't come". That cant be good for family relations. Just my opinion.