"This is ridiculous."

But despite her saying that, she continues on, practically in the same breath still:

"Yes, I can wait."

It might be seen as a way too odd and quick flip-flop, but no, both of those statements can be true at the same time! The idea of her doing something as mundane as building a sandcastle (or Duo doing that, for that matter!) is ridiculous, but at the same time, she doesn't hate it.

So yes, she will make a sandcastle with him. And not take her chance to run off while he's grabbing the supplies.