Hatabou has been Hitman Hatabou the whole time, and "regular" Hatabou is his cover.
Hatabou is a world renowned assassin, as mentioned in the episode. That is quite possibly his true form, and he's just pretending to be an innocent five year old. He could be acting cute just to get close to people and kill them. More evidence lies in Osomatsu-san. He's rich and has government ties now, and still wears green overalls as if it's just a costume for a character he has created. All of the characters who were children in the source wear notably different clothing, but Hatabou still wears green overalls. His "mystery meat" could be meat from people he has killed. He has many toys in his room, demonstrating maybe he is still pretending to be a child. This could be the reason no one came to his birthday party and other characters tend to avoid communicating with him.The Sextuplets are Demons under the control of their parents
Think about it. There’s six of them, they look exactly alike, and there must be some sort or seal that Matsuyo and Matsuzo use to get the kids to do what they want. The sextuplets do sometimes act out or be generally unruly, but the one who can stop them is—get this—Chibita, if provoked enough.At the end of Episode 10 of the 80’s series, Chibita becomes a ruler of his own planet and becomes a vessel for the dead
Chibita, after killing everyone by opening a window and blowing out all the life candles, starts the earth anew under his rule, and learns to make Oden. Meanwhile in Heaven, the people who have been killed visit Chibita, and for fun, possess him as a way to feel alive again. Maybe this was how Chibita learned how to cook?