My two sons (11 and 7) are intact, dh is circ'd. We are not a terribly modest family, so both boys have had plenty of opportunity to see dh in all his glory. Neither one has ever noticed or said a word about it. When older ds was about 6, I mentioned to him that some people are circ'd, like daddy, and some are intact, like him and his brother. He paid it practically no attention. He did ask me why parents circ and why we didn't and was very satisfied with the answer. As he's gotten older, I've given him more information, he understands it well and definitely can see why we didn't do it. Younger ds has either never noticed, or didn't say anything if he did.
It really wouldn't make a difference, as all the penises in this house look different from each other. One circ'd, big, and hairy, one intact with a fairly short foreskin (goes just to the end of the glans), and one with quite a long (half inch) overhang.
I have heard the kids (girls included) giggling about how big and hairy daddy's thing was. Now that they are all getting a bit older (4,7,8, and 11), we are more modest.
Kids really do understand and accept differences between people. The circ'd dad/intact sons "issue" has been such a non-issue for us.
My good friend circ'd her first son, and not her next two. DH is circ'd, and she has told me that it hasn't been noticed or brought up yet- her boys are 7, 4, 2. They bathe together nightly and have seen their dad's penis. No issues there either.
Hope this helps.