"Nope." He's been working hard, but he's only just starting to feel it. He's a couple of feet down, now, their height difference made less by the hole in the dirt. "If I have to, I'll use my powers. Don't worry about me." He'll give up a few seconds for this. This is important.

But Kotetsu is very focused on digging. And so he says something that, later on, even he'll admit might not be the best thing to say under the circumstances.

"You're really not good at...being a kid, are you?"

He doesn't mean it as an insult. He mostly sounds surprised, even now, after all he's seen from them.