Here's what we did with our kids. It worked for us their parents and them as children; they now do pretty much the same with their children.
Up until about six/seven, a 'restaurant kit', paper, pencils, puzzles etc. for the wait before food. Food once served, all that put away, join in conversation while eating.
Six/seven until fourteen or so: just join in with us their parents, or if out with our friends, sit at a separate table with friends' kids, ask us if they could order dessert or drinks etc., generally giggle and chat, have a good time without making too much noise and disturbing others. (That took a bit of practice at home, of course - part of what's known as 'parenting'.)
After fourteen or so, they mostly didn't come out with us. They'd be out with their friends, we with ours. Occasionally we'd go out together; we'd chat, have a laugh, do what people do when eating out.
Later, of course, they'd take us out. Sometimes together, often separately. That's fun.
Now we're grandparents. Sometimes we do the same things with grandchildren. They've been well trained by their parents so it doesn't seem strange to them to leave their screens at home or switched off.
Screens are useful entertainment for children at home or on long train journeys (with headphones of course - even "quiet" sound may disturb others (me for instance!)). But giving your children screens to occupy them in a restaurant short-changes them a bit wrt their development as people, imo. No, it's not the worst thing parents do, by a long chalk. It's just not as good as the alternative. In my opinion.
Of course I wouldn't say anything to a parent (and never to someone else's child, of course) unless, as here, asked for my opinion. Sometimes we notice other people's kids misbehaving/causing a racket in a restaurant while we're eating. We keep our opinions to ourselves. It's not usually a problem, anyway; kids will be kids, after all.
I would - on occasion do - forcefully complain to a parent whose children's screen made a noise I could hear, in restaurant or train carriage, even if it were 'quiet' (take note, OP and others!). That's a different matter. (Why is screen noise so bad? Another thread. Trust me, though, it's very bad.)