We had 4 kids in 5 years. When the youngest was 3 the oldest was 8. Someone gave my wife a stack of "Kids eat for $1" coupons. We had lessons for the the kids for 3 days, we dress "cleaned up" and off we went. The kids understood this was kind of important to them so they behaved (and were marveled by the never ending salad bar!)
It is true that most times, we went out to eat after church so they were dressed better than average, but we never had any problems.
The wife was away at a conference, and I had to make an emergency repair at work, The kids were starved by the time I got them to a Chinese Restaurant AT 6:30-7:00 ON A FRIDAY NIGHT! They got us seated right away by taking a skinny hall table and putting 5 chairs at it right inside the dinning room entrance! We were on display like a miniature "Last Supper". As adult couples had to wait to be seated, they had nothing else to look at. No less than six people stopped at our table and complimented on how well behaved the kids were!
I have been out to eat with my 5 older grandkids, It shows they all got the training! The younger grandkids are just 3, 1.5, & 1, they are supervised, and quieted as needed without disturbing other people's meal!
What is funny, was the time we're at a family steakhouse, and the oldest (12 or13) had forgotten some morsel at the salad bar, crackers of all thing, he excused himself and was politely making haste when a 65-70 year old man stopped him and ask if he wanted his crackers. My son said, "That is very kind of you sir, but they are right over there" and politely moved on. The man looked at me and said, "You taught them not to speak to strangers. Very good!" my reply was, "No, they are suppose to talk to strangers, they are not allowed to take anything or go anywhere with them, is all." (My son didn't know the man was and Elder at the church his school was in! But the man knew us.)
Kids are only human and their brains start empty, it is the RESPONSABILITY of parents (notice plural) (notice not grand parents!) to install manors and communication skills to the young. But one frazzled mom or grandma just can't do it alone! That is why our society is deteriorating! (and I think that brings us back to the OP.)
Ivan