I love this vision on Mumsnet of old people who're inclined to be miserable bastards, who've never been children or had their own children, or currently have grandchildren...and who of course all read the Daily Mail.
And then there's this...
My guess is that that older couple perceived our OP as having a lot of power (young, sexual potency visibly displayed) and also vulnerability (out with her little ones, a mother) and that, in some way, challenged them. Perhaps they felt she was also wealthier? Anyway, it made them unhappy, and that was how they reacted and rationalised their feelings
Bit of a wild guess or two there.
Sexual potency? Oh how I laughed. Power? When the power in the land (sadly) is still largely confined to the old white male you referred to earlier? I don't see his position being usurped by the fertile young woman here. He's quite safe and can continue to read his DM in peace.
Most of us are aware that while young parents may appear to have more "wealth" than we did, they also have more debt, and more need for both parents to go on working until they're - well, almost as old as us. I'm sure none of my contemporaries actually envies any of their young relatives.
In my defence, neither I nor any of my friends would eat at F and Bs...and none of us read the DM - so perhaps you're right, OP, and it's where the grumpies all hang out. When we eat out with our little grandson (2.5) we find that most older people speak to him, smile at him, and generally seem to enjoy talking about their own families. I think you've been unlucky.
Can't say I'm looking forward to that ol' basket chair they're saving for me at Shady Pines, though. Imagine if some of you have to care for me?? Eek.
I'm aware I have wandered from the OP's AIBU, but that's what we irritating old folk do. If we're not moaning and groaning, we're losing the thread.