I know what they're singing.
They're not singing any words. They're vocalizing, if you will.
Besides, they're Yoshi babies. And since the only word they know is ''Yoshi,'' they know nothing about lyrics.
There are no lyrics to the song whatsoever, and everybody should know that.
Besides, if there were, why would the developers even bother to include it?
But since this is one of those just-for-fun topics, I'll leave you to come up with your own wacky lyrics. Maybe you'll be able to come up with something better than all the other entries I've heard before.
Peaceful penguins, demented dragons and growling griffins, oh my.