Neva knows that at the end of the birthday song, you blow out the candles. That’s just how it works. So her third birthday was almost RUINED when Uncle Kyle started singing the song while her cake was still being decorated. She desperately tried to delay the song so it didn’t finish before the cake!
As many have been quick to point out, the color of this birthday-destroying dookie indicates that it was maybe even human - perhaps from a passing plane. Either way, the cake suddenly seems a little less appealing now.