How many children, actually have the money to buy themself another smartphone, or get a smartphone gifted by random people? I bet that's very rare.
In real life, it doesn't work like this for most people. In real life, kids want something and either the parents agree to buy that and then they no longer bother with that device or the kid saves its money trades something and get that device.
The idea that a kid gets an expensive device that can't buy by itself and the parents have control or what device the kid gets is a very middle-class assumption. Life isn't that perfect for most people.
Walmart has sold android smartphones of okay functionality that poor people depend on for decades.
Like, less than $100. Nowadays they have $20 ones that are locked to pay as you go monthly purchases, which is definitely worse than the old tracfone days but about the same price if you had to top up every month anyway. That's ignoring used phones.
Even poor kids get christmas money sometimes.
The commercial value of a ten year old smartphone in a wealthy country is approximately $0. It's probably not terribly hard for a determined child who goes to school and has friends to obtain one.