I'm yet to be able to successfully breed prematures exclusively, I'm still new to the sim and its different versions out there.
???: Kuccha! Kuccha! Kuccha!
"Happyness!", "Refreshed!", those are a yukkuri's common vocabulary. But lacking yukkuri say things when eating or shitting.
Furthermore...
They would eat their own poo poo and pee pee like food. Of course, those are just bean paste, so there's no need to worry about their health like in the case of animal pets.
Toilet cleaning is also easy compared to normal yukkuri. The food they scatter will be eaten as a snack after they have an afternoon nap.
Just remember to wipe them once in a while to avoid them growing mold.