Hi Otto
The Scandinavian airlines are showing a lot more humanity than the UK ones!
As for what to take I understand that their needs are changing pretty much daily. You've probably already seen it but, just in case, here's a link to the Solidarity Symi Facebook page - up to date "needs" are posted here regularly.
https://www.facebook.com/solidaritysymiYou can contact the organisers direct from there.
Looking at the latest posts they have had a wonderful response from the people of Datcha and seem to be pretty much inundated with clothing etc. I wonder if, given that nights will now be getting cooler and you will have quite a bit of space as well as weight allowance, something like sleeping bags or mats might be appropriate? I'm sure you'll get best advice from the people actually there.
I must admit that this initiative and the work being done in Symi has restored my faith in humanity to some extent.
SueO