Hi and welcome
Congrats with your chicks, and I am sorry that one died.
I really dont know much as to breeding but I am quite certain that the mother is feeding the chicks--because, she does not need to leave the nest to feed them, the male actually shares his food with her (via regurgitation), and then she will share that among her chicks. You might notice the male leaving the nest to eat more than usual. Feeding their young is a natural instinct, as is incubating eggs and raising chicks, birds are very good parents and it is instinctual for them to care for their young.
I would be guessing that they would do good on a higher protein diet at this time- perhaps some cooked egg every week or two- and added calium as your female did just use a lot on her eggs but I really dont know so please do take the others more experienced advice over mine.
I Think that your rosellas will through the dead chick out on their own, my ducks do this when eggs go rotten- but than again swallows do not through out their dead chicks.
My Best wishes to you and your new babies
