To give the aunt the biggest benefit of the doubt, I could see a situation where she may have gotten really excited about the invite and made the reservations and put down deposits before really realizing that children weren't going to be invited to the ceremony. Then getting stuck with the consequences.
Or even making the reservations way in advance and there being some sort of material change in how she felt about leaving her kids in the care of a kids club in a foreign country. In this particular situation I don't know why her mom couldn't watch the kids, but sometimes when you have small kids, things can change from day to day and it's just not possible to leave them.
If I had been invited to such a wedding with no option but to leave my kids at a babysitting service in a foreign country I would not go to the wedding at all....If I had found out the happy couples expectation after putting down deposits or making travel arrangements, I would have been furious and it would significantly affect how I felt about the couple. I don't find it out of the realm of reason for a couple to have been planning a destination wedding, and giving close family members a heads up so they can start making arrangements well before official wedding invitations go out, then deciding that they really would prefer kids not be at the ceremony, thereby leaving those who have already made reservations in a real pickle.
In this particular situation, I think it would have been much better for the happy couple to make explicitly clear either on the invitations or before those even went out that children weren't going to be invited to the ceremony and for potential guests to take that into consideration when making their decision to attend or not. This is not a time for people to rely on guests understanding the traditional rule of name not on the envelope your not invited.