All 4 of my children had to have upper palate expanders.
Only one child complained of any discomfort. All have great teeth after years at the orthodontist's office. I truly believe that I paid for his vacation home...but they put you on payment plans and are so great with children.
I believe that only orthodontists' (dentists who do braces)are the only ones who install the appliances. Usually the younger that you expand, the easier it is to expand. One daughter had the expander in first grade. You get used to turning the "key" and then seeing the big gap between the front teeth that comes with the expansion...don't worry, the teeth do move back in place...usually a better place.