I used to live in Glossop and was just talking to a friend who still lives there. She recommended a place called Two Hares, which has a really good reputation locally:
https://www.facebook.com/p/The-Two-Hares-Cafe-Deli-100091608000771/
Not sure it’s definitely on a Sunday, but worth a check. Edit: 10am – 4pm apparently, recommended by two local mates.
The other place, which is open on a Sunday, is the Pico Lounge – just opposite M&S behind the old mill in the centre of town. It’s a cavernous place, but does decent enough food, is dog and child friendly and a lot more pleasant than the likes of the stuff listed above, ime, ymmv etc.
https://thelounges.co.uk/pico/
There are also a bunch of places on the High Street and around Norfolk Square – there’s a Costa there if you like that sort of thing – but not sure what’s open and not. There was also quite a cool little caff up in Old Glossop called Shepleys, on Shepley Street, which was okay last time I was there a couple of years back, but again not sure on opening hours. Edit: not on a Sunday though.
I’m sure there are other options too. HTH, have fun.