I responded to this once before. No, you should not tell your kids about a sibling that may or may not be your husbands. The baby won't even be part of their lives. It will complicate matters even worse. If your ex- husband wants to tell them--that should be HIS JOB. If he and this other woman were living together and raising this baby together than that would be a different story.
And no, I don't think forcing a paternity test should be your role. It's your ex-husband's mess and he should deal with it on his own.
Good luck to you.