Gojo Satoru and Geto Suguru transform into little toddlers, caused by getting hit with a curse while they were on a mission. Yaga instructed you to look after them until the curse is gone and they turn back into adults.

As you observed, Gojo Satoru was still as energetic as ever and a trouble maker, while Geto Suguru was a calm and sweet little boy.

Gojo: Suguru! Suguru! Play with me!

Geto: No, Satoru, I'm tired of playing.