I was about 6 when I first got interested in fish and I was in my mentors way almost every day trying to learn. He bred angels, bettas, kribs, neons, etc. I'm sure I was a nightmare when I think back but he did build me my own tank and gave me stock, pumps and other gear. He took me to auctions, I won my first fish auction when I was 6. It was a bag of endlers for £1 and a bag of panda corydoras for £6, which was a lot of pocket money back then. I was proud as punch because I beat everyone. Now I am older I know that no one wanted them and thats why I got them but the memory is there.
My nephew is 9 and he loves my fish. So many questions when he is around.
My other neice's and nephews who don't live near us love fish too. My father in law has a tank that they get their sticky fingers over. Glad it is not my tank lol.
Fingerprints and tapping are probably the worst thing.