An alternative one could consider is that the ISP could DNS-block sites for "kid SIMs"). On the OS part, this would require protection against picking alternative DNS, but that's already standard. Other "anti-circumvention" features might be required, such as factory reset.

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.