Why does my world seem to be getting smaller?
Solution 1:
These are probably in the same shape as the "chunks" in your world, and typically form when a chunk has been loaded incorrectly or corrupted. As animals can still enter it it's probably just an error loading your world.
Restarting your world should work (as in either restarting the server it's hosted on, or restarting your game). If you're playing with your cousins see if they can see the issue too. If all of you can not go into the areas it may be that that chunk is corrupted and you may have to restore a backup (if you have one).