DC's no longer able to "connect" on minecraft

I'll start by saying that I'm decidedly un-techy, so please bear with me!

My dc's used to play on minecraft on their iPads and were able to connect and go into each others' worlds. They are suddenly unable to do this and are quite upset (as am I. It used to keep them quiet for ages ).

I've checked they're both still connected to the wifi. I've also checked the multiplayer options on minecraft are on. I've tried switching them off and back on again. And that is the extent of my repertoire of tech related options.

Anyone else had the same issues and been able to resolve them?