Fledlings missing from courtyard

Some beautiful little blue tit fledglings came to my juliette balcony to the feeder on Monday. There were two - three buzzing round the courtyard and chirping loudly. 

They were last in a tree in the courtyard on Monday evening and I have not seen them since. I am fearing the worst and praying they are ok. There's a local cat a few streets away and a magpie that hangs around so I'm hoping they are safe.

How likely is it that they have moved on to another area? They visited my feeder a few days with parents that day.

Also I have a scruffy little blue tit visiting my feeder every 2-5 minutes. I assume they are feeding the young and have a big nest? Is this safe- I have put out peanut butter and suet balls which they love. I've tried leaving fruit and water out but they don't touch it so I'm worried about the babies dehydrating.