Welcome aboard to BYC family gr8 people tons of information, loads of fun, enjoy shake your tail feathers some times you feel like a nut, some times you don't
OP has stated she is in Australia and it's very hot there. There is a chance her chicks expired from that.
They need fresh water and you teach them to drink by tapping the bowl like a hen would do. If they don't drink, gently dip the tip of the beak in the water so they know what it is.
You can't see water unless it is really gross, so they need clues.
Very young chicks wear out easily and may fall asleep standing up till they fall over.So it is best to have a shallow container of water with some clean pebbles or marbles in it so if the chick lurches forward it will not drown.
Are the chicks eating their feed? Silkies especially may need a finer crumble. Be sure none are bullying the others away from feed and water. If you could show us how you keep them we could offer specific advise.
G’Day from Far North Queensland Ronichook Welcome!
I am so very sorry to hear of your loss
I am also sorry that I am not able to help with a reason; I only have experience with hatches under a broody hen and none whatsoever in keeping chicks in a brooder.
I recommend popping in to the Aussie Thread where there are some NSWelshers who may be able to offer advice regarding the brooder and the current temps.
I too am swaying more towards heat stroke. How hot was it on the day you got them? Did you travel far with them? They may just not have drunk enough water in the current high temperatures.