Original sentence: "Snowdrops are the first flowers to pop up when snow melts."
Corrected sentence: "Snowdrops are the first flowers to pop up when the snow melts."
Why? Adding "the" makes the sentence sound more natural because we mean the snow that was on the ground. You can also say: "Snowdrops are the first flowers to appear when the snow melts."
Simple explanation for a 6-year-old: Snowdrops are little flowers that come out right after the snow goes away, so we say "the snow" to talk about that snow.