They don't have to have those. Depending on your definition of "kids", most people on HN I imagine are not giving their kids phones, laptops, or tablets at young ages (maybe less than ~13?). And if they do, I imagine the devices are somewhat locked down and monitored.
I think the more technologically literate a person is, the more wary they are of unfettered access to it for children. Hence, preferring a stationary desktop where use can be supervised.
I agree desktops are best, and they are what my kids started with, but there is a lot of pressure to give kids phones.
For example, where I live, the cheapest (monthly) bus tickets require an app, so kids need a smartphone to get to school (or their parents have to pay a lot more for daily tickets).
There is a lot of social pressure on the kids too. There are lots of activities that have either moved online or are organised online. Lots of ways to get left out.
laptops and phones mage that a lot harder.