You want to take your kids out but they're playing so incredibly nicely. You know if you hazard into their 'pretend' territory, you will ruin this idyll, you will turn it to tears and tantrums...just by being you.
You want to take your kids out, but they're being complete pills. Throwing tantrums, fighting, making a big deal out of nothing, intentionally setting verbal traps for you, etc. You can't move them into public in this state.
If you happen to have nothing scheduled in stone where you absolutely must be somewhere at a certain time, do not stress over when you leave. Let it happen, don't force it to happen. (You can do this with a scheduled event, too, if you start early enough, and have a backup plan for if they actually get in the car at a reasonable speed.)
For instance, this morning, I told the girls I'd take them rollerskating. Now, usually this would mean me moving immediately...grabbing the skates, putting on socks and shoes, harassing them out the door, let's get a move on.
But, since we had nothing else to do today, I told them, and they got excited, but when they didn't move toward getting out of the house at all, I waited. I let them play their games. I did some work. Then when they brought it up again, I immediately set down my work and got their socks and shoes. By the time I got out of their room with the supplies, they were playing again. I let them play. I picked my work back up. When they brought it up a third time, I put their socks and shoes on. I went to get them out of the house, but they had begun playing again. Same thing. About an hour from when I first mentioned it is when we finally got out of the house. With no muss, no fuss, and everyone on the same page.
I've said before that you only have between 5 and 30 seconds from when the children decide to do something to actually do it. The difference in this pointer is that if you happen to miss that window, don't sweat it, don't force it. Wait for another one to come along.