MB here. I really appreciate your sensitivity to this, and if you were my nanny and I knew you were this thoughtful about it I don't think it would bother me.
If my nanny were encouraging this, it would feel different. But my kids are coming into a pretty imaginative/creative age and we are learning that we (parents and nanny) have to make sure we're all on the same side. If we start believing or getting worked up about everything that comes out of their mouths we'll all be in trouble!
You might just mention it to the mom and let her know that you're sensitive to this issue and not encouraging it. Basically just let her know it's the kids driving it, not you, and give her a chance to let you know if it bothers her in any way and if so, how she'd like you to handle it. Or to say "Oh, that's totally fine - don't worry about it at all."