Love in your mind produces love in your life. This is the meaning of Heaven. Fear in your mind produces fear in your life. This is the meaning of hell.
Children are happy because they don't yet have a file in their minds called "All the Things That Could Go Wrong." They don't have a mind-set that puts "Things to Fear" before "Things to Love." Unless we can be like little children, we can't enter into the kingdom of heaven; unless we can be like little children, we can't be happy. Children are happy because they don't have all the facts yet.