I'm sorry that you feel criticised. I don't think that posters are meaning to do that. Sometimes there isn't a good solution / a solution that everyone is happy with to a problem with dementia. People with dementia lose empathy and find it difficult to change their behaviour which is why posters have talked about keeping your mother and her great-granddaughter apart.
Perhaps the timing of visits would make a difference. The behaviour of people with dementia often gets more difficult later in the day (a phenomenon known as sundowning) and so visits earlier in the day might be better. Similarly, shorter visits might be better than long visits or overnight visits; childcare for a whole day may not long be possible. Some older people, with or without dementia, can't cope with young children for more than short visits.
Depending on your mother's abilities and interests, perhaps getting her and your granddaughter to do a joint activity (e.g. a craft activity) together, supervised by another family member, would be a good way of them spending time together. Going on outings might work too.
Is it possible that your mother is jealous of her great- granddaughter because she (naturally) gets a lot of attention or is the centre of attention? If you can't have a meaningful conversation with her about this then you might need to make sure that your mother is always getting a lot of attention when her great-granddaughter visits.
I noticed in the case of an elderly friend with dementia that she didn't cope well with several people visiting at once and all talking together. I think that she couldn't follow the conversation, couldn't contribute (because she wanted to talk about what she wanted to talk about, which often didn't relate to the topic of conversation which changed quickly - plus she could ramble on in a long monologue) and didn't like not being the centre of attention. She could get quite shirty.
It's a difficult situation. As I said above, sometimes there is no good solution. I hope that you can find a strategy or approach that works so that your mother and her great-granddaughter can enjoy spending time together.