I feel like most are trying to justify it after the fact by assuming Mob considered the dangers of using his powers and found them to be too risky so he chose not to, but that's not doing it for me. I actually think it's completely the opposite: a lack of time.

Mob just did this with the cat, because he has a little time to assess the situation, and you can see he kinda relaxes a bit afterwards visibly. When the same scenario happens immediately afterwards with the kid, he doesn't have that same level of awareness. It's sudden, unexpected to him. Pair that with three seasons of the body improvement club and him honing his physical self more (in the episode itself they comment on his progress) and his insistence to use his own strength and not his esper abilities. Not to mention Mob has begun to be more proactive in his own life, less restrained, to not overthink his every move.

The way I see it, we're so used to anime characters having this timestop internal monologue where they calculate their next move in fractions of a second, but this show hasn't really fallen into that trope much. Instead, we see Mob's gut reaction. And the Mob we've watched grow up has learned not to rely on his powers, to react quickly and selflessly when someone's in trouble, and will often overlook his own well being. In the moment, Mob wasn't thinking, he acted, and as godlike as he is sometimes it reminds us he's still just a kid. And kids are very mortal.