My child drops things over and over, and knocks down or breaks what they just built. It's frustrating.

Both are important learning. Dropping is exploring cause and effect—"when I do this, it falls"—and knocking down or breaking is trial-and-error experience. Instead of stopping them with "you dropped it again..." or "you just built that!", watch over them within safe limits, and they'll come to understand cause and effect and grow the ability to try again and again.