Honestly, your MB should mind her business. That nanny is still around those 3 kids, probably because she does her job, they’ve never been seriously hurt and she has a good relationship with her employers; letting one person take care of 3 under 3 means the parents trust her and knows what she’s capable of- or they’re idiots and neglect their children as well
Your MB can have an opinion but she can’t tell you who to talk to. She can’t prevent that nanny from going to the park on the same days as you and honestly, how would she know? Are you suppose to ignore her when you see her? Absolutely not. Just say okay and continue doing your job!