Because why say it otherwise? Facebook could've easily left all of that out and just said "Introducing messenger for kids! We're making sure it's carefully curated and safe for your children" etc. etc.

People would've speculated that it was being used for ads, sure, but it wouldn't have caused a major outrage. I'd bet significant sums of money that most (not all, obviously) parents don't really care about their children being advertised to or used for market research - I mean hell, TV was doing that way before FB was a thing. Most parents probably worry about more obvious things w.r.t. children chatting online: Bullying, talking to strangers, etc.

Saying "we're not scraping childrens' messages" and then turning around and doing exactly that would be such a monumental PR disaster if it ever came out. I don't think FB is quite that stupid. They're clearly focused on the long-term.

Another possibility is that they're actually just legally barred from scraping messages sent by children, and this is them putting a PR spin on "we're complying with the law!"

So basically, I just don't see a lot of benefit to promising not to scrape messages if they really want to do that.