This is not about children or adults, it is about royal families and monarchies. Of-course, if you invite a friend round to dinner and they never show up (or worse still, they show up and never repay the compliment - of which both situations I have had the misfortune to experience), then indeed the relationship changes and you most likely do stop putting out a hand.
However, the situation with royal families is a different matter altogether because the private and professional elements of their lives merge in a very different way from the rest of us. A royal funeral or royal wedding is not simply a family or friend event, it is a state, semi-state or national event of a country and in such circumstances, personal feelings etc are diluted with protocols, conventions, traditions and political/constitutional requirements.
It reminds me work colleagues - you sit next to them all day, chat and gossip around the photocopier, have lunch/coffee together and attend meetings with them and even go on team-building events etc etc. But very rarely would you go to their mother's funeral or invite them round for Sunday lunch.
Queen Margarethe gave an interview a year or two ago in which she was asked how she thought the Duchess of Cambridge was coping with the media. HM initial response was that she would never comment on a colleagues professionalism (or words to that effect).
When I invite a friend round to dinner, they either come or they don't and if they don't come, they don't send someone else instead. When you are royal and invite another royal from a foreign country to an event, royal convention allows someone else to go instead - either a lower ranking royal or an ambassador for instance and frankly, it is not for us to question that because it is as it has always been.
Believe me, it would be great if members of the BRF spent more time with other royals, Queen Elizabeth out shopping in Copenhagen with Queen Margarethe - I'd be happier than ever to see that!
Now, whether or not the British Royal Family seem too snobbish to attend foreign events is a matter of differing opinions and not a matter of general fact - indeed, members of the BRF may have no inkling at all that some people may think they are snobs, they may also have no idea that they are the most "famous" royal family in the world because once you are "known" through out the world, (which they will be aware of), how much or well-known you are becomes insignificant. WE see them as the most famous royals in the world, but ask for instance a Dane to name someone who is royal, they will no doubt say Queen Margarethe or CP Frederik!
It is not a case of the BRF wanting to be isolationist if it is an inherent trait in themselves to be isolationist. Being perceived as being isolationist does not make one a snob. Whether they are or not is neither here to there, because the closeness, the friendliness and the bonds they have with other royal families is as it always has been.