Although, I have made numerous pairs of longies in my cloth diapering days, I searched Ravelry for a pattern that incorporated a monster grin. I found one and was delighted to see that the smile was accomplished through short row shaping and the eyes and teeth were merely duplicate stitch. Yarn was easy as she didn't want them to serve as a diaper cover. The project was a quick knit. He is only 6 months old which helps.
I am very pleased with the result. My duplicate stitches could use some work, but all in all I like the result. I hope he (or at least his parents) loves them.
Project info can be found here: Gus is a Monster Longies