Hi Pippaclaire and welcome to the forum ...
You are probably very right to be concerned about nestlings overheating if left in full sunshine ... young birds and animals can find it difficult to maintain a safe body temperature if they are not able to get themselves into some shade.
However, erecting a strange structure such as an umbrella so close to the nest has undoubtably spooked the parent birds.
It is only natural that their greatest instinct is for self preservation ... if they survive, they can have another brood. That said, they also have a strong instinct to protect their young ... and the older the nestlings are, the greater the will of the parents to continue feeding them. Parent birds are much more inclined to abandon eggs or very young nestlings, than offspring that they have already invested a lot of time and energy on.
So really this situation requires close monitoring to see if the parents are still feeding the nestlings. If they are, good ... cause no more distubance to the nest or family until some time after the young birds have fledged and have left the nest for good.
If the parent birds have made no appearance or attempts to feed the youngsters, then the brood will need to be taken into care by an experienced wildlife rehabilitator if possible, to ensure their survival. If the parents have indeed deserted the nest, then the nestlings should not be left more than 12 hours into their normal feeding day (starts just after dawn).
It is not practical to just leave food for the nestlings ... they can't feed for themselves and won't until quite some time after they've left the nest.
Be aware that approaching the nest again in daylight might well trigger a panic reflex in the nestlings and make them flee the nest, possibly injuring themselves in the process. If the parents are not about to feed them, they won't survive. Likewise, if they aren't close enough to fledging when they jump the nest, the chances of surviving even with the parents around are also considerably reduced.
There is a list (not exhaustive) of wildlife rescue organisations on this website at:
Animal Rescue Centres in the UK | Wild About Britain
Make some calls and see who is able to help you ... the closest centre on this list may well know of someone closer who isn't listed.
Best of luck ... please keep us posted/