>How come ad supported TV existed for decades without destroying children's mental health?

Well, there's at least a few reasons this is different than the current situation.

1) It's expensive to make a TV show, it's free to do a fortnite dance or eat a tide pod and post it to several websites. The amount of low-effort, low-quality, probably-harmful content on TikTok or whatever is exponentially more than low-effort, low-quality, probably-harmful TV shows/ads.

2) The availability is on completely different scales. TVs are (basically) fixed in a specific place. Phones are, for most people, within arms reach 24/7.

3) What can be shown on TV is significantly more regulated in most parts of the world, and control mechanisms by governments are more robust (pull a broadcast license, etc.). It's harder to take a website (or TikTok, whatever) offline than it is to pull a harmful show/advert off of HGTV or whatever your favorite channel is.

4) TV is not specifically tailored to the viewer to produce the most amount of happy chemical.