I say keep feeding your daughter like you are. Your son's interest is very natural and innocent. My niece had the same interest if it makes you feel any better. My son was 2 when I had my daughter and he was curious at first, but that has since faded. He really wanted some milk so I pumped some and let him drink it. He really liked it and I would pump some for him occasionally. It's way healthier for them than cow's milk anyways. I don't do this, but there are plenty of women, especially in other countries, who still nurse their children at age 3 so I wouldn't worry about it. My son is 3 and I still nurse in front of him. Sometimes while I'm nursing he'll make a comment very matter-of-factly, "Those are your boobies to make milk for baby Kylynn." And I just tell him he's correct and that they made milk for him too when he was a baby.
Nursing is a beautiful part of life and not sexual whatsoever. It's adorable and fascinating to watch animals nurse, so I completely understand his interest. I have to admit that before I had my own children I was very fascinated by the whole process (I still think it's the coolest thing ever and I love the human body and how it functions). My SIL was pumping in front of me once and I tried to be discreet about my fascination but I was simply amazed. It's a wonderful science lesson, and I think it's important for children to see the human body in a natural, healthy, nonsexual way.