Sometimes Marius will regress and just be completely out of it and dissociated, just wants to be held and chew on the little baby-safe toys the other mechs get for him. He’s not really safe to leave alone because he gets fussy and will try to do things with his adult height and weight that just… do not work because he’s a baby and he will get hurt. But he doesn’t want to play and he can’t get himself around well or follow instructions.
To solve this issue, Raph makes him a little baby sling so anyone (usually Brian, Raph, or Jonny) can carry him around and talk with him as they go about their normal tasks. He’s perfectly content with this situation and always comes out of being regressed feeling very safe and relaxed.
CARRY THAT BABY AROUND!!! Even if he's out of it, he can still register the fact that he's being held, that he's safe, even if he isn't quite registering WHO'S holding him. He's not really processing what they're saying, but he can tell that he's safe and loved. Do NOT leave him alone because he WILL get cranky and scared.