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So I don't think you did anything wrong, and I'd even go so far as to say selling to anyone but the renters would be tantamount to leaving them to the wolves, but let's lay out some points
They've lived there for over a decade - by all rights except property rights, the house is theirs. At the same time, you played by the rules and built up the slight amount of generational wealth it'll take to give your children a free pass into the rapidly shrinking middle class
From what you've said, your position is not unethical or exploitative - if anything, owning a few properties and renting them at below market value does more good than harm
It's entirely unfair that you get to essentially tax them for the right to survive, but that's the only offer on the table. Ideally, you charge a rate near or below your costs over the long term and only make a profit through equity - they are already paying for the mortgage and you take care of sudden large expenses like appliances and repairs... Double dipping and also squeezing them for an ongoing profit is where it goes from potentially fair to profiteering. You get long term wealth, they get stability
Now where it gets dicey is with the pressures of the situation - management companies are getting real cozy with corporate property investors. When they inevitably ratchet up the prices or drop you to focus on bigger fish, will you pass the costs on to them or look at other options? If someone offers you something way over market value, will they have a seat at the table? When you pass it on to your children, will they end up left to the wolves then?
I think you should put thought into these questions, maybe attempt to get to know your tenants enough to feel emotionally connected to them (if they're amenable to the idea obviously).
And personally, I think you should just do right by them and hold out. Our system is untenable, we've reached the limits of easy growth and our economic systems are cannibalizing themselves - if you provide stability at fair rates until we reach the point of transition, I think at that point you can look back and say you did right by them
Tax them for the right to survive? It's called exploitation, and it's the name of the game. If you can't beat them join them.