Those are great photos, and your nieces are certainly living life to the full.
My brother climbed Kilimanjaro a few years ago, and said it was one of the highlights of his life. The hardest part wasn’t his lack of fitness, but the lack of oxygen at altitude make breathing a battle of the mind.
He has a great respect for the guys who climb Everest, considering that Kilimanjaro’s peak is about the same altitude as the Everest base camp.