I don’t have kids, but I use “your mom” when talking to my niece or nephew about my sister.
The problem I run into is with referring to MY mom, their grandmother. Using “my mom” is weird. And “Mom” doesn’t work either.
Using “your grandma” also seems weird (maybe because I don’t think of her as a grandma but as a mom?).
The kids have a family-specific nickname for her that I use more often than not, but it’s based on a childish mispronunciation of “grandma” so that’s also a bit weird. It is, at least, unambiguous. (It’s also the term my sister uses when speaking to them.)