It's a reassurance statement. Maybe the child has some separation anxiety? Maybe it's the parents with separation anxiety?

It's akin to telling a child that mommy and daddy will always be there for them.

Of course, tragic things can happen in this world and lifetime and tomorrow is not guaranteed. Yet, when it comes to words of affirmation and comfort, it's completely typical an expression.

Much like saying I love you to your kids or spouse or loved ones as one heads off for the day in different directions.