It's not that I don't fully understand your question.
However you state (she has had wet diapers at home and I think she is controlling herself and about ready to learn to go to the potty.)
having a wet diaper and then stating you think she is controlling herself and ready to learn potty training are contradicting each statement. If she were ready to start potty training, she wouldn't have a wet diaper. This is what was confusing in your question. So of course a rash will develop if she is left in a wet diaper for a long period of time.
I don't know your Granddaughter, however usually around 18 months is when some children show interest in potty training all the way up to 3 years of age. So going by 14 months, is early.
So it's not a joke here Hun, I had just wanted to confirm.
Thanks
Lindie