There's no real difference, both are grammatically acceptable. Only :
Je viens vous sauver = I come to help you
Je viens pour vous sauver = I'm coming in order to help you / so that I can help you
In the first sentence, you're just saying that you're on your way to help them ; in the second one, you rather highlight why you're coming, you insist on the goal, the objective.
"Venir" does not implement the future tense. To say "A small child will save them" in French, you would say "Un petit enfant les sauverait." This sentence above literally means "A small child comes to save them."
As an aside, the sentence "Un petit enfant va les sauver." also doesn't translate to "A small child will save them.", as the tenses don't match.