Anicca: Impermanence
Conditions change, and no arrangement holds still. While the child was breathing the situation was open, and David pressed into that opening with everything he had; when the breathing stopped it closed, and he turned with the change. To keep fasting past that point would be holding an umbrella up long after the rain has stopped, which is what his household seems to want from him.
Upādāna: Clinging
Clinging is the grip that wants a passing thing kept. Seven nights on the ground with untouched bread beside him were a hand closed hard around a single outcome: let the child live. Once the thing gripped is gone the grip has nothing to close on, so the hand opens on its own, and washing and eating follow without any struggle to release.
Parideva: Lamentation
Weeping and crying out are grief's loud phase. The tears while the child lived were aimed at someone, a plea to the God who might still relent, and they end when there is no longer anyone to plead with. The servants expect the wailing to swell now, since afterward is when wailing is supposed to start, and its absence is what they cannot read.
Sīlabbata-parāmāsa: Grasping at rites and observances
Treating a practice as owed, apart from what it does, is grasping at form. Their question, "What is this that you have done?", assumes mourning has a required shape: fast afterward, weep afterward, louder than before. By that measure David has run the order backward, and the broken sequence is what stops them.
Upekkhā: Equanimity
Steadiness when nothing further can be done is upekkhā. "He rose up and ate bread" is that steadiness in the plainest act available: a body that needs feeding gets fed, and the ordinary day is allowed to start again. Composure here is not coldness, since the love that kept him on the floor for a week is the same love that now stands up and asks for food.