Oobleck (cornstarch & water)was poured down my drain, maybe 4oz., by my child and is now stopped up. Now what? Can I unclog it or will it require a plumber?
If it's only 4oz, the quickest fix will be to fill the sink until you can get your toilet plunger submerged in it. Then plunge repeatedly. Once it starts draining rinse the starch down with plenty of water.
If that fails, take the p-trap off and give it a good cleanout. Likely the starch settled down there.