What elements of foreshadowing are readers given throughout the story that something very bad is going on in the nursery?

Every time George and Lydia visit the nursery and see the veldt scene playing, their deaths are foreshadowed. They find George's chewed wallet and Lydia's bloody scarf on the floor of the nursery. The lions even seemingly come running at them as if attacking them early in the story, foreshadowing their end and sending them screaming from the nursery:

And here were the lions now, fifteen feet away, so real, so feverishly and startlingly real that you could feel the prickling fur on your hand, and your mouth was stuffed with the dusty upholstery smell of their heated pelts . . . and the sound of the matted lion lungs exhaling on the silent noontide, and the smell of meat from the panting, dripping mouths.
The lions stood looking at George and Lydia Hadley with terrible green-yellow eyes. . . .The lions came running at them.

The whole look and feel of the veldt is ominous and makes the parents uneasy. It is hot, dry, and predatory.
Further, the children lie to their parents about the nursery. They even change the scenery on the view screens to encourage the parents into thinking they are not always watching the veldt. They behave coldly to the parents, foreshadowing the disregard and lack of feeling that will allow them to lock the parents in the nursery to be consumed by the lions.

There are numerous examples of foreshadowing in "The Veldt." Early on in the story, when George and Lydia are in the nursery, they notice that the lions are chewing on a carcass. They're curious to know what animal's being eaten. This foreshadows their own gruesome deaths; it is they themselves who are being eaten by the lions.
Another example occurs when George and Lydia hear blood-curdling screams coming from the nursery. Once again, this turns out to be a premonition of their own brutal deaths on the veldt. As does the discovery of an old wallet belonging to George and one of Lydia's old scarves. Both of these items have been torn up and are covered in blood. This hints at the unpleasant fate that awaits George and Lydia in the nursery towards the end of the story.