They're still pre-school, so maybe they were overwhelmed with all the excitement. Saying "good-bye" for them in this type of situation, especially if it keeps them from getting hysterical, and also because it's not something that happens often is a perfectly acceptable way of handling things.
My son used to *always* cry heavily at the end of a day with a lot of excitement. He's older now and doesn't do that any more, but when he was little, he needed the release because he couldn't process all his emotions yet.