I went into Petco a couple weeks ago and there was a mom with her two kids looking at goldfish. Comets to be exact. Me, keeping goldfish, and looking at the snails in the next tank over, started up small talk.
"What kind of setup do you have?" I asked, after explaining that I kept fancy goldfish (to a look of bewilderment....non-fishkeepers never understand why some one would keep a bunch of "12 cent fish", lol). She respnded with "a 2 gallon bowl". I asked if she had kept goldfish before, and she said they had bought one from walmart that died yesterday after 2 months. The kids ran off to mess around the store and while they were away, I took advantage of kid-free time and explained stunting and why goldies need large tanks and filters. Knowing the kids would be devastated if they went home without a fish, the mom and I compromised on a betta. Still not an ideal setup, but she did water changes every other day at least, and bought betta pellets, and was seriously contemplating upgrading to a filtered 10 or 20 gallon at some point in the future if her 6-ish year old kids ever actually got more into fish when they were older.
You may not be able to make them completely see your side, and that betta may not be going into the hilton of fish homes, but it's much better off than a goldfish, and everyone got what they wanted. That mom acted like I freaking saved her life, too. I think when you actually have experience keeping that particular fish, and when it's a conversation versus a lecture, people are wayyyyyy more likely to listen to you. The Petco fish employee just sat around too, until she noticed I was leading the mom to the betta's and pointing out things to make them happy.