Pls explanation or theories <3

So, I've been rereading WHA lately and only now I realized something that has been bugging me for a while. Right at the begining when Quifrey is about to cast his spell to fix the carriage, he asks Coco to protect the door while he is drawing the spell so no one will come in. My question is, why did he chose Coco? Like not as in, "she's the main character" or as in "bc she's the one who was really excited about it and kinda wanted to help".

Why did he chose her when she's just a kid? Of course an adult would be equally as bad, probably having a higher chance of wanting to look through the door. But a kid is curious and a lot of the time, bold and mischievous. For real Quifrey didn't have that thought into consideration? Plus, I feel like the consequences of having memories erased is way worse in a kid than in an adult, just for the simple fact that they are someone who barely started living life. And in Coco's case, he unintentionally I think, got Coco in her mom's situation which is even worse. Sometimes I think it would be cool if this was a major plot point that will be revealed later in the story but I also think is just Quifrey's mistake. I think it was in volume 2 that he apologises to Coco for getting her in this situation.

Idk, I'm just curious about people's thoughts. Maybe this was already clarified somewhere in the manga and I don't remember it. In any case, I want to know if this was just Quifrey's mistake.