It's like having a "no trespassing" sign and a fence around your pool. You still might be in legal trouble if a kid hops your fence and drowns, but you're vastly better off from a legal perspective than the alternative of not having any barrier whatsoever.

That comparison hinges on having to click "yes, I'm over 13" being more of a fence+sign than a "tiny" text saying you should only use the website if you are 13 or older. I'm sure some lawyer will argue that's the case - since I'm not one: Sounds rather flimsy.

It's certainly the case, especially legally.

Here's Chatroulette's login screen:

https://imgur.com/a/PV3sT0r

And from Archive.org, here's how Omegle's looked when the girl who is now suing them joined the site. "Tiny text" isn't an exaggeration. The call to action to start a text chat is a 200x50 button -- the 'don't use if you're under 13' text is 0.75em font:

https://imgur.com/a/QpFBJ15

It should be obvious to everyone, not just 'some lawyer' that the former is more of a barrier than the latter. There's also the concept of overt acts in many statutes - lying to a website by clicking a button that says "I'm over 18" when you're not demonstrates that you read the disclaimer and disregarded it, where you can plausibly claim you never saw the copy when it's just legalese on the bottom of the page.

I expect at least some kids to be scared off by this.

The BBC article above states that Omegle is being mentioned in 50 pedophilia cases in the last 2 years. If 20% of kids would be scared to click "I'm older than 13", that would be 10 cases fewer.