Schedules, Naps, and Throwing the Rule Book Out the Window

Both of my boys are asleep right now because neither got a nap today.
My mom called this morning and asked if I could take my little brother to a friend's house down near where my husband works and then I could surprise my husband for lunch. Well, it's been a long time since I've been able to have lunch with my DH due to gas prices/little children so I said yes. So Evan kind of got a nap in the car, but Will didn't sleep at all from 8am this morning until I put him to bed at 7pm.
So here's the question: is it ever alright to mess your kids' schedules up like that? To "take the rule book...and throw it out the window"?
Personally, I think it's okay to do on occasion. I know that little one's especially thrive on schedules and knowing what's coming next in their day. I try to stick to roughly the same schedule every day and I find that really helps me too when I'm having a sleep-deprived day.
On the other hand, it makes for very grumpy, tired children.
I still think it was totally worth it to see the look on DH's face when I showed up with lunch <3