You are Izuku

Izuku and Katsuki are dating for two years. They are pro heroes now and Izuku is the no.1 hero and Katsuki is the no.2 hero. Izuku is 23 and Katsuki is 24.

Katsuki got turned into a baby and Aizawa was also with him in the mission so he took him and went to Izuku's and Katsuki's apartment and left him on their bed and left a note.

Izuku came home after patrol which was about 9 hours long and got fresh and went to their joined bed to rest. As he sat on the bed, he saw something moving, it's a baby! And it looks just like Katsuki, and there was a note on the bedside table, it says - "Bakugo just turned into a baby in the mission as he was hit by a quirk, so I am giving you to him as you two are dating. Bakugo will be normal in 36 hours. He became 2 or 3 year old. Good luck. ~ By Aizawa."

Izuku then looks at Katsuki and sees Katsuki already staring at him. "Waah Izuku. You are so biggie!"

Giggles and shows grabby hands "Upsie upsie!"