It is alleged Letby, 33, attacked the prematurely-born infants during a day shift at the Countess of Chester Hospital’s neo-natal unit.
Child M “dramatically” collapsed in his incubator in the unit’s intensive care room at 4pm on April 9 2016, the court heard.
His brother, Child L, also deteriorated beside him at “pretty much the same time”, prosecutor Nick Johnson KC said.
Jurors at Manchester Crown Court heard Letby messaged her mother, Susan, at 1pm about the Grand National, due to be staged later that day.
She wrote: “Is dad betting on the Grand National? If so can he see which are greys and put a bet on for me please x”
Her father, John, gave her the names of three horses including the eventual winner Rule The World, the court was told.
Letby’s mother texted her at 5.25pm with the news that Rule The World had triumphed.
Two hours after the unexpected collapse of Child M, Letby messaged a friend: “Work has been shite but … I’ve just won £135 on the grand national!! (horse emoji).”
The court heard that Letby had moved into her new home – near to the hospital – earlier in the week.
On a group WhatsApp post to friends just before 2pm on the same shift, she wrote: “Sorry guys, mad busy four days in work. U can come to mine if you want to. Just need to unpack first.
“Haven’t got a spare bed yet tho so cant stay unfortunately. Looking forward to a catch up.
“Got magnum prosecco and vodka woop. No disco ball but sure we can manage”.
Following her Grand National winning bet, she tells the same group chat: “Unpacking party sounds good to me with my flavoured vodka ha ha. Just won the Grand National!! (horse emoji).
The Crown says Letby poisoned Child L with insulin and also injected air into the bloodstream of Child M.
Letby, originally from Hereford, denies murdering seven babies and attempting to murder 10 others between June 2015 and June 2016.