I once heard someone suggest that this should be on the OS level and I'm slowly coming around to the idea. There should be some kind of OS level flag that can easily broadcast to products that a child is using the device. No ID verification required, the parent is responsible for setting it, and all downstream applications are regulated to respect it, If applicable to the product. This removes the incredibly invasive…
Building this functionality into an operating system is an extremely slippery slope to even further centralized corporate and government authoritarian control of people doing what they want with the personal computing devices that they own and possess.
For all the protections that the ISP cannot reasonably provide, one can consider creating a nation-wide (or EU-wide, or US-wide) official label, that device makers can obtain if they meet certain requirements. Parents can them make their choice. We have that for food etc., so why not for internet access mobile devices. Because from my perspective, the main problem that needs to be solved is that parents cannot monitor 24/7 what their children do on their phones.
Yet another solution is to create a top level domain for kids, restrict kid devices to that domain, and allow sites enter this domain only if they meet defined criteria.
Oh, and prevent ISPs and phone makers to sell 'for kids" products at a higher price. People can whine all they want about free market, but it's to protect our children.