My approach to writing lyrics, and I'm sure it would work here, is to work out what I want to say first without worrying about meter or rhyme or number of lines or anything. So here you could ponder on what are the situations / examples you can think of about being young, having no worries, only dreams.
Stuff like:
I look at pictures of the world, of space, of faraway places, of great men and women, and I know I can be one of them, go anywhere I want.
I see young lovers (albeit older than the youthful narrator) and know one day I will find the greatest love.
I hear someone crying and I know that I'll be there to dry the tears of sad people.
I look at the stars and I wonder on the wonder of it all, and I know someday I will know what it all means and it will be perfect, it will be Heaven.
Once you cam imagine that young person with no fear, no worries, just dreams you can then start to work on what filtering those dreams will mean.
Just ideas...