To some it may not be an issue, but I believe that on some level every parent knows what I am talking about. The Kids. When you have one, it is not so bad because there is just the one to worry about and you know where and what they are doing. But, when there are two or more it's like they KNOW and they team up together. With more than one child they are very sneaky and you don't always have that eye on them. One may be sleeping and the other(s) may not; they take their sleep in shifts. When the kids come to be a certain age it is like they go into Secret Agent mode and they come up with little plans to difuse the intimate times that mommy and daddy try to share.
Sometimes I feel like a Secret Agent myself (I go by code name: Tigress), I have to engage the kids in an activity that will capture their attention for at least 15 minutes, then I race to the "Rally Point", keep it quiet, put myself back together, enter the room calmly and fix a snack for the kids.
Planning...it all takes practice and if you get into a good routine without the kids defeating it, you will be a happy mamma. Eventually when they get older, they will be out of the house doing after-school activities and so on, then there will be no need for all of this racing from point A to point B. But until that time comes, it is actually pretty exciting to have a certain window of opportunity that could be closed at any moment.
Sending the kids to Grandma and Grandpas house is always a good fail safe too.