Time to cover up?

Whenever my dad sees my daughter running around naked in his house, he will freak out. He will quickly yell for her to get dressed immediately. However, in our own house, there are only the 4 of us. So we are ok with the kids changing clothes in front of each other or running around naked while mummy chases them around to get dressed.
Sometimes, the kids will come into the room while I am changing my clothes as well. And with the kids alone at home with me most of the time, I will leave the bathroom door slightly ajar while I'm in the shower or doing my 'business'. At times, the kids will just open the door and talk to me, in the middle of whatever I'm doing in there, as if it's no big deal walking in on mummy sitting on the toilet. If my dad knows, I think he'll freak out even more.

So when should I stop being so 'open' with the kids? They are 4 and 6 years now. I can still remember some early memories from when I was 5 years old myself. So I definitely do not want my son to remember seeing mummy in the bathroom when he grows up!

But the thing is, if I do not keep the door open when I am in the bathroom, then it will be difficult for me to hear what they are doing outside. I need to know that they are not jumping on the couch or fighting with each other while I'm in the bathroom. Of course, I can just warn them not to do anything naughty while mummy is not in the room, but how often would they listen? What if they got hurt and I couldn't hear them because the door is closed?

What do you do? Do you close the bathroom door and leave your kids outside alone?