What amount is abnormal with fascination of anatomy?

My dd is nearly 6, and DS is just 4. They are both very aware of their own bodies and other people's and have no issues with nudity. we often talk about others bodies being their own and we shouldn't ask to see bottoms and other private parts..

I just opened the photos in iPad and found 20ish photos of ds's bottom, penis, and a very blurred version of what I think was dd's bottom taken yesterday. Part of me sees it as nothing but kids being kids. I was not aware they actually had the iPad in their room, which I normally don't allow.

But part if me just isn't sure when their fascination with others bodies is too much?

To me sounds normal, I mean think of how many adults think it is funny to take photos of bottoms etc. I wouldn't see it as fascination more as isn't this funny naughty sort of thing. In the days before Internet I am sure lots of people found photos on film cameras of bottoms etc that kids had taken and didn't think twice. I remember being at a wedding not long ago where there were disposable cameras on the table, the bride and groom said there were alot of bum, boob and penis shots that people obviously thought were hilarious to take - to me this sounds no different.