The first two sentences are fine. There are various ways to say the third one. Perhaps:The children and I went on a picnic. Unfortunately, it was cloudy through the day.
The children who had been looking forward to it also seemed upset.
If it would had been fine ("fair" can also be used?), the blue of the sky("blue" should be no article or "the blue"?) might have been beautiful.
- The children had been looking forward to the picnic nd were unhappy that the sun wouldn't come out.
- The children had been hoping for a sunny day and were disappointed in the sunless sky.
There are other possibilities.
Use "fair" to describe a cloudless day. Say:
- If it had been a sunny day the blue of the sky might have been beautiful.
You could also say "fair day" I suppose, but it isn't used all that much.
3. METEOROLOGY not stormy or cloudy: sunny or clear, and without much wind