I feel less angry about it when it’s someone young, not working, etc.
I think, if they're very young - probably far too young to be able to do this kind of thing - I understand much more, when they're too little to be able to grasp how the figures work in reality; e.g. if they sell lemonade, cakes or whatever to make money for a local charity, but they don't factor in that their parents have had to spend money to buy the food/drink/ingredients, and likely as much (or more than) they're actually 'raising' for the charity.
As for those who aren't working, has it not occurred to them that, if they were working, they may well not have the time to go off on jollies, BUT they would be able to give to their beloved charity themselves?!
I agree with PP: they never seem to care about volunteering in desperately underprivileged inner cities in their own country; or indeed in countries that are grindingly poor and happen not to be particularly pleasant, or even dangerous, countries to visit. Whilst obviously not in the same league as places like Norway or Switzerland, Ecuador is in the top half of the world development index. Why aren't they going to 'help' in the very poorest countries, like South Sudan, or Somalia, or the Central African Republic?
I'd be inclined to ask them this directly, if they approached me for funding for their holiday. If they'd find this rude and crass, they started it!! At the very least, I'd want an in-depth chat about how they were specifically identified as the best person to meet this need in a far-flung land - and what skills they alone were able to offer, especially when compared with those of the people with experience of living and working in the country. Yes, they would find it embarrassing and awkward... but again, they were the one with the audacity to ask me to pay for their holiday!
I do agree that it's the height of arrogance: the very suggestion that some random Brit is more valuable to their community projects than a skilled, experienced local. Surely nobody can't see through this clear scam anymore? Even if they did have the skills that are needed to help, it's highly unlikely that there are not already similarly-skilled people already in the country, who will not have to take the costs of travelling halfway across the world and back out of the precious funds available.