Why did mishkafayim change to mishkefey, given that mom and dad arent wearing the same pair of glasses...
It isn't meant to be plural, it's the double construct. One pair of glasses is also משקפיים. Like it's plural in English and French,as a pair.
In construct state משקפיים becomes משקפי.
If you want to specify what glasses (like sunglasses) you put them in construct state. Mischkafaim becomes mischkefei (both words being plural)
There's Hebrew for you. :-) Yes, this happens in certain circumstances in the construct state. The rules are a bit obscure, so you'll have to play it by hearing.
Doesn't שמש mean helper, as in Hanukkah, the shamash is the 'helper' (middle) candle?