A very early nest I think and sadly 5 chicks were abandoned after being monitored as per BTO guidelines for three weeks. After 8 days of feeding with just the female doing all the hard work. About 5am Sunday morning the mother left and another bird perhaps great tit or sparrow squeezed into the nest. Due to early morning sunlight it could not be seen what happened for about 30 minutes.
With the internal camera view one chick was still alive after 12 hours. No parent returned and 24 hours later all chicks are deceased. On cleaning nest box there was no sign of injury to them
it was very noticeable that there were very caterpillars or similar, they were mostly being fed aphids and some fat ball food maybe.
Having seen what they were being fed I think the mother possibly gave up out of exhaustion or was predated or spooked by the other bird.
I'm afraid if you have a house sparrow going into another species' nest box, then nothing good will happen. Not ignoring the food comments and I'm not a fan of people feeding 'just the adults' as adults take food back to the nest regardless of who it's targeted at. I don't know your setting and location, but unfortunately, as people are encouraged to put up nest boxes, many get sited in inappropriate locations where there is next to no natural food source. Could be that an adult died, though very difficult to sex them.
Might be worth seeing if the 'invader' got in hours earlier.