Every single one of those is common place in the digital world these days, and this is no exception. By getting these devices in the hands of kids for less than the cost of the device, you can affect what services they choose to use in the future (by making them already familiar with your product by the time they can choose for themselves) and setting them up to live in your walled garden and making them pay a premium to stay in (see also, the Apple model).
Chromebooks very intentionally push people toward Google drive and Google one for subscription services, and all of the rest of gsuite for data mining. I'm not saying chromebooks are inherently this evil master plan, but don't discount just how much they do push profitable services for Google.
I never said there was lock-in, they just do the same thing every other tech giant does nowadays and makes their "products" all integrated to steer people toward them. Chromebooks have native gdrive integration in the file manager. Gsuite apps all come pinned and pre-installed. It, for a long time, had no way to run a browser other than chrome, which itself has all sorts of integrations through your Google acct. That all serves to steer people toward staying in the Google ecosystem and avoid trying to reach out of it if they don't have prior motives toward that.
Staying == lock in. You can have a chomebook without paying a cent to Google after the initial purchase, and leave at any point without hassle. I have no idea how is that a loss leader or anything of the sort.
The truth is that Google doesn't sell them below cost and not everything is a conspiracy.
The hardware manufacturers (because that's separate from Google) are not producing hundreds of thousands of Chomebooks at a loss. Chromebooks are cheap because they don't need much hardware because ChromeOS is lightweight.