"An apple a day keeps the doctor away" : Stop eating junk food! You kids are going to end up in a hospital. Remember that an apple a day keeps the doctor away.
If you eat healthy, you won't get sick.
"Alive and kicking" : I heard your grandmother got out of the hospital and she is alive and kicking again.
To be healthy and active despite health problems.
"Feel on the top of the world" : I've been exercising and eating healthy, so now I feel on top of the world.
To feel very well.