When you get a prescription for these lenses, and it’s time to get the lenses made:
Ask the opthalmologist / optometrist that writes the prescription if they will put this phrase on the prescription - “No tolerance for axis”. Without these words, it’s perfectly OK for lenses to be delivered up to 7 degrees wrong without them needing to remake the lenses. 7 degrees wrong on a -16 would be criminal bad optometry, and a 2 year old would be unlikely to be able to vocalize just why exactly he doesn’t like his new glasses. “No tolerance for axis” will cut this behavior off at the knees - the lab tech will know not to mess around.
Ask to speak directly with the optician in charge of the lab. You want the most experienced technician they have (Not a salesperson, just to be clear - you want the guy who actually gets his hands dirty grinding the lenses.) No offense to the salespeople, but they’re more likely to be interested in making a sale - and a commission - then making sure your nephew’s lenses are as flat and thin as possible, and these things can be lost in translation sometimes since the lab techs rarely deal with customers face-to-face.
Tell him that these lenses will be worn by a child, and ask him to recommend the frame that will best accommodate the lenses. They’ll be able to look at the prescription and visualize what the uncut lens will look like, taking into account the cylinder/astigmatism and such, and believe me, a lab tech that’s worth his salt will immediately know what frame will be best.
Ask him, challenge him, even… to get those lenses as thin as possible, and the edges as smooth and polished as possible.
Don’t buy into the idea that the more expensive an optical shop is, the better quality the eyewear will be - the opposite is usually true. Places that charge $1000 a pair don’t make nearly as many orders (and often don’t even HAVE their own lab) and their techs don’t get as much practice with the unusual prescriptions. Steer clear of the dirt-cheap Medicaid-focused type places as well, and zero in on a mid to large chain. Eyemasters, Lenscrafters, PearlVision, etc. - look for a place that has a lab on premises - although they still may need to send your nephew’s order to a special-order lab, they’ll likely still be the ones doing the finishing/polishing.
Make sure that the staff takes plenty of time doing the final fit when the glasses are delivered. Have your nephew shake his head vigorously a few times to see if they slide down. Don’t quit until they’re comfortable and snug. Because if they slip down, the optical center of the lenses won’t be centered on his eye anymore, and a 2-year old will most likely not understand why his vision just went all blurry and know to push them back up his nose.
There, I’ll get off my soapbox now - best of luck with your nephew and I hope he does great with his glasses. There’s something almost magical about when the really young kids get their first good look at the world. It’s awesome to see. Kudos to your sister and BIL for getting on top of the situation BEFORE he’s 8 and can’t read the blackboard!