When a child asks you how the babies are made

Sperms and eggs, just say that. When they ask what sperms are you tell them the most scientifically baffling answer ever if they're young kids. "Sperm are male gametes, reproductive cells that contain half a set of chromosomes that can be joined with a female gamete to form a zygote." You don't have to explain sex really if they're young, just explain the microscopic aspects of reproduction until they're ready to learn about the rest :)