Also know nothing about Minecraft[1], but could this be a WiFi client isolation thing? If the PC is wired, and given a typical consumer router with client isolation enabled the iPads would be able to connect to it, but not to each other. Depends on what sockety things Minecraft is doing, of course.
[1] My inner 12 year old very much regrets that it wasn't invented before adult me didn't have time for such things.