What does it mean to be with me and you when you grow up?

It means growing up with each other forever. "When you grow up, I will accompany you all the way" expresses the desire to accompany each other all the time, which means to accompany each other all the time. Accompanying all the way means that two people have the same friendship, or the feelings between two people, which can make you understand each other, tolerate each other and accompany each other in your later life.