Separation agreement legality

I recently filed my separation agreement and my wife came up to me yesterday saying she was going to take our child and give custody to her father, who is a stepfather actually. My question is can she do that even though we already have it filed? And what are the implications if she tries? Could I sue her for breach of contract if she does, and therefore possibly get complete custody of our child?

Courts can always modify custody in the best interest of the children. If she tries to “give” custody to her step-father then you may have to file a custody action to get the children back.

So if she does go through and give him custody would I have to pay child support during our separation period, or would that only be effective after the divorce was finalized?