Mobile data usage is doubling year on year.
The costs of keeping up with this and delivering a decent user experience mean that unlimited plans are completely uneconomic to sell.
There is a massive customer demand for unlimited data, but no-one is prepared to pay a realistic price to allow even the mobile networks (who have the lowest cost to serve) to offer anything. The only way they do it is by imposing fair usage rules and selling on the basis that plenty of unlimited customers are overpaying and should be on a cheaper tariff.
Why do you think Three is trying to buy O2? They can’t make it work without a market-busting economy of scale.
OP – just buy an unlimited broadband product, set your kids’ phones to pick up your wifi by default, and make them pay any mobile out of bundle charges. It’s the only way!