Our daughter's PWS covers part of her eyelid, too. Her doctor uses a small, cupped stainless steel shield during laser treatments. It is shaped to fit her eye, with a "stalk" sticking up out of the back of the shield so that it can be easily removed after treatment. The part of the shield that touches her eye is covered in something like silicon to make it smooth and slippery - so no pain or damage to the eye. Plus, the doctor gives her a numbing drop in the eye right before inserting the shield. It just takes a second or two to put in the shield, and it slides right out after the laser treatment is over.
These shields come in lots of sizes, including really tiny ones for newborns (which we've used with our daughter). The Laser Skin and Surgery Center in NYC (where our daughter has been treated 12 times) may be able to point you towards the vendor they purchase their eye shields from.
Blessings to you and your child!