Why is it necessary to add the "a" in "yo salvo a los ninos"? What does it mean if I leave the "a" out of the translation?
This is called "the personal 'a'" in Spanish. Basically, you need to add an "a" when the action of a verb applies to people (or if it's personified). For example, it is "Yo veo a Luis" (I see Luis), but "Yo veo la silla" (I see the chair).
Is this the case for all people in all situations or is it only for people that you know?
All people in all situations. It's also the case when you are personifying something, like when you refer to a pet.
Yo lo veo a Luis. Isn't it necessary to add "lo"?
No, you only would use it with a pronoun: "Yo lo veo a él" versus "Yo veo a Luis".
Thanks. I have always wondered why sometimes an "a" is used.
Muchachos and niños are interchangeable in my part of the world.
El salvo a los niños, pero no los niños ingleterras.