Relaxed lunch/dinner on Christmas Day in Lisbon

Unfortunately, you are likely not going to find what you are looking for. Christmas is the only day that virtually everything closes in Portugal. It is very different than in the states where you can still find groceries and restaurants open. I’ve been told that some of the Indian restaurants are open but I’ve not checked that out.

Try googling Indian restaurants close to your accommodations. If that doesn’t work, I’m afraid it’s going to be a fancy hotel restaurant.

As usual, Kathryn is right.
Especially as it is your first day, why not plan for dinner at your hotel? If they are not serving, ask them to recommend the nearest place that is. Where are you staying?
I don't think you want to be prowling around looking for that "authentic Portuguese" place on Christmas with a 10-year-old and jet lag.

Much regard,
Dennis

When I was about your son’s age, my family ate at a Chinese restaurant in Europe on Christmas Day. It was obviously memorable for me - but it was also the only thing that was open! I agree with what others are saying. Your options are definitely limited on Christmas Day!

Could try contacting Nicolau Lisboa to see if they will be open. Looking at their facebook page, it appears they were open Christmas day last year. I've eaten there a couple of times when I crave more healthy, vegetarian options. It can get busy, but fun casual place.

https://www.ilovenicolau.com/en/

Actually, the last time I ate there was after landing from an overnight flight and prior to taking a train to Porto. Their smoothies helped settle my stomach after suffering on the plane from motion sickness.

Thank you all for your replies. We are staying at an airbnb, and I had already asked the host about restaurants on Christmas; however, I was hoping to find some helpful recommendations from seasoned travelers here excatly because I don't want to "prowl" around. Also, I never mentioned I wanted "authentic Portuguese." Nicolau looks exactly like what I was hoping for! I've contacted them to see if they will be open Christmas Day. Thank you again!

Since you are staying in an Airbnb, you might want to ask the host if they would be willing to buy any essential groceries for you to have in the flat that day and night. There won’t be any mini markets (or larger supermarkets) open for anything you can’t live without until the next day.

Asking the host for basic groceries is a great idea. Thank you!

We are also arriving on Christmas Day, staying at a hotel (Avenida Palace), and they've been extremely helpful - have found us a dinner reservation for the six of us in walking distance to the hotel. Just FYI! Good luck and have fun!

Thank you. I've found a couple of casual restaurants, and am looking forward to being in Lisbon!