I don't watch a lot of creepy/spooky stuff, so my recommendations come from a fairly limited breadth. That said, I recommend a few things that many might lump under "kid movies" (I prefer the more accurate label "family entertainment") since they tend to be perilous and unsettling without being outright violent, gory, or generally miserable.
Coraline (2009) - A young girl, dissatisfied with her home life after moving to a new town, stumbles upon a dark, parallel world. Therein, she finds solace in a parallel version of her mother who is not what she seems.
Paranorman (2012) - A young boy who can speak with the dead learns that a witch who was executed by the townspeople hundreds of years ago will soon return to seek vengeance upon them.
Over the Garden Wall (2014) - A mini-series focusing on two brothers who find themselves inexplicably lost in a forest teeming with fell beasts, witches, undead, and unlikely allies. I watch this one every year around this time. Cozy yet spooky at the same time.
In my experience, you don't even need the dedicated Jellyfin client. Just opening it up in a web browser works out of the box, so that's potentially one less thing to download/install/manage for the clients.
That said, I've never tried to access Jellyfin from an iPad/iPhone/Mac so it might not be as seamless as my experiences on Android/Linux based devices. But I imagine they'd be fine; just test it out before you hit the road.