I think one teaches respect for privacy very early on. That doesn't mean "shame" over nudity but it does mean gradually adding layers of sensitivity. That means door knocking and waiting for someone to say "Come in" (vs. knock and barge in, which many kids do). It means closing the door when using the bathroom (home or public).
I would think that your 13 year old absolutely doesn't want you to see her nude, so I'm guessing that she doesn't much want to see you that way. Your 9 year old might not care about you seeing her, but that day is rapidly approaching. Also, I think it's very likely that, for some time now, she's commented to her friends about her mom's nudity or what breasts look like after a baby is born or after a woman is 30 or 40 or whatever, and so forth. Then those kids are saying, "You know what? Tiffany said her mom walks around nude all the time. Tee hee, tee hee." So the rule for me is, if I don't want my naked body to be the topic of conversation on the school bus, then I keep it to myself.
A friend also commented on the awkwardness of a kid barging into the bathroom while she was attending to her period - and how she had to answer questions in the midst of what her son had already seen. That's an important conversation that needs to be held over many years and at greater levels of detail, but it should be held at an appropriate time and not because the situation was rushed by someone coming in during the removal of a tampon.
I think it's also a good idea to teach proper discretion so that, if they should live in a house one day in a crowded neighborhood, they are thinking about what's visible to someone walking by. The same thing occurs in a locker room if someone is too near the door and that door happens to swing too wide. Your current home may be isolated, but you do want kids to think about keeping personal things private. The images they see now will dictate their behavior later on.