No such thing as an 18 month old bully. They don't have the issues yet that make them want to make themselves feel better but making someone else feel smaller or less worthy.
So, do not back down. If the mother speaks to you again, ask her to be extremely specific. Say, "Don't label the child, describe the incident. Don't criticize me, just give me the facts of the behavior." If you missed that her child and your child were together, you can thank her for the information and say, "Thank you for telling me." Nothing about what you're going to do about it or why it happened. Nothing.
If you don't get anything satisfactory, say, "Well, perhaps your child and my child should not play together." Or, since they weren't together, say "Then it's a good thing they haven't been near each other, as my daughter has been playing with her cousin." (It may be interesting for this woman to know that these 3 kids are not all your bio kids - as if that's her business anyway.) If she rants about your parenting, suggest that neither of you are watching kids while she vents. Then move on.
The purpose of play is for kids to play and to learn to get along. That is not achieved by helicopter parenting, with a parent supervising and correcting and directing every breath and every step. If that mom wants to live that way, that's her problem. You could also say that "Kids play and things happen. If that makes you uncomfortable, perhaps this isn't the best place or the best time of day for you and your child."
Don't engage her further. Let her know that you have heard her. And try to figure out what makes you so uncomfortable. Are you afraid she will cause a scene? That she'll yell at your daughter? That she's right? That she'll report you to the management? Knowing the answer will help you decide how to proceed.
No one can make you feel guilty without your permission.