As soon as the level starts, go straight down into the gap between the snakes! The rat and snakes are synced up so that by staying close to the snake in front of you, you'll avoid the rat's first pass, and can grab the bottom cherry. Then go back into the snake gap and to the left side, and you can wait for the rat to pass before heading back up to safety (grabbing the key on the way).
Hope that helps, and thank you for enjoying the game!