Cirno and Daiyousei are playing one of those hand games that elementary girls play. The song they sing is their own version, although in Japan there are also many different kinds of the same song.
I added the fourth line in the notes on the first panel to complete a sing-song, poem-y feel to it. Literally translated, it would be something like this:
Right next door, The grandma and grandpa, Eat sweet potatoes and fart, With the power of fart, They fly in the sky.
Th, the sky!Hm?If someone really flew in the air by farting, it would get all stinky, wouldn't it?The hag is...!!Tewi!!The grandma and grandpa right next door;
Eat sweet potatoes, fart, and fart some more;
With the power of fart they begin to fly;F, Farts!!La— la la la—
la la la la
(rest omitted)Tewi!grandmaThat was fun— ♪Yup, that's right—flying in the sky
