I'm here!!
Oh boy. Where do I start ...
(let me preface I am just reading about this and do not have all the info I need pertaining to the crew.)
1. This popcorn mess is laughable. Seriously. I have seen sooo much worse. Here's an example and yes, those are upturned seat cushions. Don't ask me why, because I don't know why. My personal eye roll

was crushed up Cheerios from kids stomping on them. If only I had a pic. People are disgusting. They just are.
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2. Do we really know the Flight Attendant forced her to pick it up? And getting the captain involved? Spilled popcorn is not the captains priority. I don't know of one captain that would get involved, this has nothing to do with safety. I can't imagine any flight attendant calling up to the cockpit to tell them about this.
3. Personally, I would have never said a word. Like I said, people are disgusting. The only time I made someone take care of their own trash is dirty diapers and when a man took out the Sky magazine and used it as Kleenex. Yes, a grown man.

I made him walk with me to the galley and throw it in the trash receptacle after I opened it for him.
However, I did not work for this airline and I don't know how this all came about. My best guess, and it is just 100% speculation ... the mom and children treated the crew like the garbage they spilled on the floor. So with that attitude the flight attendant gave it back. I still find it suspect she "demanded" the mom pick it up, but you never know. Would I have done that? No. But if she and her children were absolutely awful I would have suggested to work with us and help pick up the mess they made.
4. My airline did have cabin service come on and clean the plane most of the time. They do have the hokey sweepers, but sometimes on short turns and non-service stations it was left to the flight attendants to tidy. Let me add, when it was the flight attendants job we did NOT have a hokey sweeper - they don't leave that on the plane (you don't have anything not necessary to add weight.)
5. It is hard to separate personal opinion because I ALWAYS pick up my messes. (a few weeks back we had the hotel room discussion, I leave it clean.) Heck, even when my water broke in the doctor's office my husband was on the floor with paper towels picking up the fluid before the OBGYN came in. We always picked up after our child, or we had her pick it up when she was old enough. Personal responsibility, such a novel concept sometimes.
6. Here's where I have my PSA. I know, I know, I know. Flying sucks right now. The airlines are flying jam packed with minimum staffing. However, please realize this is not the frontline employees fault. You are blaming and taking your anger out on the worker bees who have ZERO control over it. Once a crew member signs in for their trip, they are at the mercy of the airline. Yes, we have a published trip, but crew tracking can and will reroute if needed. We have no control over why a plane is delayed, why they scheduled only 30 minutes to turn a 767, weather, other passengers behaviors, etc. No doubt there are nasty flight attendants out there that need to retire. But there are also those that are really good but are being pushed to their limits not only by their airline but by the surly passengers too. Flight Attendants #1 responsibility is safety. You never see it, that is why it is taken for granted. You see the service side, which really isn't why we are there but should be treated importantly too. You can blame the airlines and rightfully so, but blame the behind the scenes VP's, Executives, and CEO's. They are the ones approving how it is running. I can certainly tell you it isn't the frontline employees because we are complaining too. How can we do a good job when we don't have proper tools? (And FWIW, my airline was non union. So telling me to take it up with my union meant nothing.)
Ok, off my soapbox.
With that ridiculously long novel said, I cannot truly pick a side. The tweet in itself sounded so entitled that is why the debate started. And my guess is if no passengers came to her defense or even assisted her then that speaks volumes about that families behavior.
I still cannot fathom that being a demand. If so, her union (since United has one) will deal with it. My guess, it will be nothing.