There wasn't any derision in Dumbledore's voice, so Ben didn't think he thought that a bad thing. In fact, he seemed amused more than anything, though he was sure that Dumbledore was the type of wizard who could present any outward facade he wanted. Ben had a pretty good read for people though, and in this case he was certain Dumbledore was amused, and hadn't minded at all.
"If we ever get to perform for a live audience, you'll have to come, see your work on a larger scale," Ben said, grinning. "And we learned all sorts of things in Charms, but unfortunately this wasn't one of them."
He grinned, slightly sheepish at being caught out, and did as asked so he could shake Dumbledore's hand. "Yes, that's my dad. He's been the Healer at Hogwarts for several years."