Pretty simple. I didn't pay attention and didn't get this until the Forest world, but I'm pretty sure you can do it right away.
Click on the emotes symbol (the blue one near bottom right) then just click the 1st one to the right of center for the emotes, a face with a open smile and squinted eyes. Rinse and repeat. You can use the same Keflings, would also be good to do it before you put them to work. Use the music emote to get them all near you and then do it.