If she is potty trained and within 30 days of her 3rd birthday then she will be allowed to go in.
Once in awhile they make an acception if the birthday is close and they club is not full. But usually they follow the 30 day rule. ALL kids must be potty trained or else you have to use the nursery.
mom x4,grandma x4
I am going to guess that she will not be able to join the club until she is legally 3.
disney monitors the ages of all kids on board and only allows so many on per age to be able to accomodate all the kids in the various programs. they do have the flounders club where for a fee you can drop her off for care.