Hi J.,
There can be a number of reasons that a tummy distends but there is a very common one today. The processing, the preservatives and the pesticides in our food supply now are causing the distention of stomachs. Weight gain goes straight to the tummy area instead of all over the body. It's considered a form of malnutrition. People with a good, healthy immune system gain weight evenly. Those with a weak immune system gain weight in their just their thighs or just their stomach, etc.
If your granddaughter eats a lot of processed foods or goes to McDonald's or other fast food restaurants often, it could be something that simple. It's not a time-critical concern that you have to worry about, but it is something that should change if that is the issue.
If her mother had it, then she probably had or has a health issue also. Not necessarily something that most people would call serious, however, but something that should be attended to. Many conditions today did not exist 50 years ago, even 20 years ago.
Hope I helped. If you would like more detailed information, I'll be glad to talk. Like I said, it is very common today, and can be remedied quite simply. Let me know.
Regards,
M.