At first Inigo doesn't quite realise what Roland is doing. He feels the sudden touch on his cheek, looks at the other with surprise-- and then after a moment or two it does dawn that Roland must have cleaned his cheek, leaving him blushing slightly, but happily. It's definitely the kind of thing a parent does, huh.

.. but that was just enough of a distraction that Inigo realises he completely missed Senku's instruction just now. And asking about it is sure to give him away, so his brain quickly kicks into gear, trying to think of another way out of this--

Right.

"Ah, Roland, why don't you show us how you do it first?"

GOT IT. No one will suspect a thing. Inigo can just copy what he does!