DD has 'met' her paternal grandmother 3 times, and DS never has.
In light of recent events, they will never have contact with her because she is abusive.

When they are old enough, I will also tell them that she is 'mean'.

You have to make the decision for your little one.
Is it worth exposing her to someone who may 'turn' and become abusive or not a great influence on your child's life?
Is it better to not expose them to start with, so that this person never has to be 'taken away'?