Not better to link both device in one network, but of curse with wire share disk in windows pc, and add it on libreelec :).
What is the point to have a low power device that you can control from the couch, if you need to have a computer on to use it ? Also ethernet cable or PLC isn't a option. The speed I get on wifi matchs my fiber speed, but of course is not enough for a real time 4k playback.
For your kids I suggest you fix both the hdd and your box to the back of the tv (if you user hdmi-cec for control)