Did you help them yesterday?
The auxiliary "Do" in that form "DID" happens because it hints something in the past (simple).
So, the affirmative form would be "You helped them yesterday.
And the answers could be:
"Yes, I did. "
"Yes, I helped them yesterday."
"Yes, I did. I helped them yesterday." (a more bookish/academical way - so more likely - but the other answers could be used too.)
As The Student pointed out, to answer DID you have to use a past tense, so "help" being a present tense wouldn´t be a proper answer there.