I can sort of see her side a little.
If she doesn’t like children then really her DH and her SS (your DH) should be keeping your DC away from her and trying to stop them bothering her.
Maybe she felt they were ‘getting in her face’ and felt saying she doesn’t like children, I.e. please keep them away, was her last resort?
I know they are your DC in their own home but maybe she thought she was doing the best thing by visiting you but just felt overwhelmed by the DC.
Maybe your FIL wanted her there so put pressure on her to go even though she isn’t happy around children?
Maybe she had a lot of miscarriages or Still born babies and now says she ‘hates children’ rather than have to go through the pain of explaining and your DC (or any DC) are just too much for her?
Maybe she didn’t want you to say she was standoffish because she never visited or was creating a divide in the family and doing a ‘pick me’ dance because she didn’t want to be around your children so sucked it up and visited you but found it too much?
There are so many reasons she might have behaved the way she did.
She absolutely shouldn’t have said anything in front of your DC!
I can understand you love your DC more than anything and think they are absolutely wonderful but she doesn’t and she doesn’t need too, that’s not a reflection on you or your DC. You need to try and not take this as a personal attack and realise it’s her problem.