I wouldn't have any issues either way, regardless of their age. When they are older, I'd ask their reasons for doing so if the request seems out of character for them out of curiosity, but being older, they probably have a reason for choosing to do the activity.
When they are younger, the most likely cases are pure childhood curiosity or play - not much more. I believe it's adults that misinterpret their motivations to "mean" something.
As a young girl growing up, I hated traditional "girly" things. There are old home movies of me getting dolls and clearly by the look on my face I was not impressed. I didn't like dress up, I didn't like tea parties. I hated dresses and make-up, still do. I had more fun riding my bike with the neighbor boys playing cops/robbers, making mud pies and building forts. I didn't grow up "gay" as people would often think that type of behavior would imply - I'm definitely hetero. I simply had a very logical/analytical mind and was interested in science, math and building things. I thought the "boy" stuff was more fun and active. Just suited my personality and interests. Perhaps the girl wanting to do boy things will be the next astronaut, scientist, inventor. Nothing wrong with that.
For the guy - maybe he's simply highly creative, an artist who wants to experiment. Maybe he saw a show like Face Off on SyFy and wants to be a monster builder or makeup artist to create wonderous and scary creatures for film/TV? Maybe he's doing it on a dare? Maybe he has an idea for a costume for Halloween or some movie his friends are making? Why does a color or "nail polish" (or anything) define the sexuality of a boy? Maybe he wants to be an actor? Maybe a scientist and heard something about nail polish and wants to see for himself? I still recall an old movie called Kindergarden Cop where a parent was concerned their boy wanted to play with dolls...turned out the reason was the dolls got the boy close to girls in the class and he used the dolls to look under girls skirts (aka, he was acting very much like a boy). Growing up, my brother wanted to play the flute (traditionally seen as a "girl" instrument). The reason he wanted to initially was due to seeing the old Bugs Bunny cartoon with the dummer and the fife - he wanted to play the fife because he thought it looked cool. Sure, he was teased by peers in middle school...but come high school...when all the other girl flute players turned into gorgeous women...then his friends were completely jealous that my brother was sitting in the middle of all these women who wanted to be his duet partner, while they were stuck surrounded by a bunch of other guys.
I think often times adults forget the general curious nature of children and project their own "fears" or assumptions onto such requests. Just let the kid be a kid, and be available to answer any questions they ask.