KS10
Slip on indoor footwear - preferably that can get wet if no en suite bathroom.
Floors in hall are yucky, especially kitchens but you don't want to faff putting proper shoes on every time you go to get a cup of tea.
Similarly, a small bucket type thing ( even better if has drainage holes) to take toiletries to bathroom. There won't necessarily be anywhere to put a normal toilet bag.
Clothing to get to and from bathrooms. Won't necessarily be anywhere to keep full outfit dry but especially in mixed halls you don't necessarily want to be skipping along in just boxers or a nightie.
Extension leads ( check regulations)
Bed throw - usually only comfy place to sit and keeps duvet cover clean. Cushions along the wall can make it more like a sofa , but not an 'essential'.
Basic eating & drinking utensils and if any cooking provision a small saucepan and mini fry pan ( beans/ eggs on toast, heating up soup etc) & can/ bottle opener.
Hot & cold drinks & biscuit tin ( preferably full). Canister of dried fruit & nuts.
Fold up clothes airer ( useful for damp towel - best if clothes tumble dry as much as possible).
Door stop ( so they don't end up shit off from passer by. Usually no allowed officially but if they are in the room & awake it's usually ok.
We sent ours with a very basic (£15 type) spare mobile phone for emergencies.
Worth checking the type of bed if you can. If storage space under it's good to have crates or at least those stuff 'bags' that fit under. Space is very limited
Possibly a cheap flat pack bookcase. Not essential but if floor space it can provide extra storage. Might be something that needs to wait until they get there.
Antibacterial wipes in case the bathroom / kitchen are really disgusting.
Tea towel!
Probably enough 'basics'

Oh, if they are going north of where you live make sure they are prepared for the weather. They might need gloves, hat and a proper coat before Christmas. And some sensible footwear! One of DCs had flatmates who only had ballet pump type shoes and they got deep snow in the first term....