When to start feeding cucumbers.

I don't start feeding mine until the first flower appears. There will be enough goodness in your compost until it needs potting on

While they're small they don't need feeding - wait till they start to flower as sarajane says 

The seed leaves often shrivel and die, so don't worry about that as long as the more are growing ok.

They have gone rapidly downhill in just a couple of hours. I expect the seed leaves to be an item of the past by this afternoon. I wouldn't worry about seed leaves dying off if there were a few true leaves, but there is only one true leaf showing and the second just a small bud type thing. If the seed leaves drop off, they will look absolutely stupid. Why both doing the same thing at exactly the same time? They have been happy for two weeks, and nothing has changed. All the toms I set in the house copped for exactly the same fate. The common denominator looks like the compost.