Tip #1: Move Nap Time
I know, I know, naps and schedules are important. But as the weather gets warmer, those gorgeous golden hour sunsets get a bit later... often pushing the limits of bedtime. I personally love shooting and sunset, as it's the prettiest light. Just this once, I'd suggest letting them stay up a bit late to play under that beautiful sunset and adjust nap time so they're ready to play!
Sleepy, grumpy kiddos usually don't smile much for photos, To make sure your kiddos are smiling and happy, move their nap back a bit so that they can stay up a bit later than usual.