Getting the POE hat in there somewhere, somehow, would be pretty neat.

The PoE hat - at least the existing one as we know it - won't work for the 400. Besides the 40pin header it also uses another 2x2 header behind the magjack, which is where it receives the input from the ethernet jack.

Oddly, this board spots a separate ethernet transformer which does support PoE, but there's no circuity to tap that, and nowhere to add it. Which begs for a modification, but seems like missed opportunity.

PoE seems off for the target audience of this device. In all likelihood the most prevalent mode of network access is going to be via wifi.

Oh totally. And I can see why they'd want to keep the component count down. Their regular keyboard is $17, and the 4GB pi4 is $55. So if you just duct-taped the pi to the keyboard, you'd already be $2 (plus tape) over the RRP of the 400.

The bit I don't get is why use an ethernet transformer at all, especially if they're not reaping the benefit from it.

Agreed that it’s a missed opportunity but a Pi with a GPIO hat attached and receiving power over PoE and in hand of a kid doesn’t sound particularly great