This is unfortunately a very common occurrence at this time of year. Is it possible for you to put them back in the same location in some kind of semi encaged enclosure out of the reach of cats where the parent would be able to access them to feed them but they would be free to escape? It's probably too late for this event, but perhaps we bird lovers should scout round car boot fairs for old bird cages or similar containers and hang them in trees off the ground or on house wallbrackets to rescue such birds in future.