Well, if the concern is they might miss out on revenue from a small child accessing their site and they are not a child specific site then perhaps they should be pushed out of the market one way or another.

No, what I mean is that adding a lot of laws to let websites be legal (age verification, GDPR, etc.) adds a huge burden and that prevents new small websites from even existing.

All I am proposing is that websites that may contain adult material add a header. That's hobby level effort. Small companies using a CDN or platform provider can either add the header in the CDN, check a box if this were to take off or open a ticket asking to add the header.