A very good friend of mine used to shower with his sons when they were small (like, after mowing the grass, etc.). The boys are 6 years apart, so these instances occurred separately.
Each son eventually noticed that his penis was different from Dad's (though not really specifying how) and asked about it. My friend and his wife explained that boys come in two forms, ones with "open" heads and ones without.
When they told me this, I really let them have it. I thought this was a terrible way to approach the issue, a guaranteed way to cause anxiety in the child. What boy wants to believe he's a different species from his father?
I asked why they didn't simply say when Daddy was a baby he had an operation. My friend said, "I don't like the fact that I had an operation. Why would I want my kids to know that? They think I'm perfect. I'd rather tell them there are males of type A and males of type B. I'm an A and they're a B. Sometime after they learn Santa Claus isn't real we'll have the talk about circumcision."
I told them this still sounded like a bad idea, but at least the boys would be prepared for high school, where they'd be the only intact boys in the showers.
It turned out their private high school did have daily showers and yes, they were the only intact boys -- I learned this from the older son, who is now almost 21 and told me that to this day his parents have never whispered a word about circumcision and just let him think he was an oddity; they never even told him his brother was intact.
They didn't even mention
to him any of the 4 excellent books about circumcision/being intact I had given his parents over the years and which they accepted gratefully. Not a peep. The younger son just started high school last month and I can only imagine what he's going through; he's the one who always succumbs to peer pressure.
My advice: in age-specific language, tell 'em the truth early and probably more than once.