In "The Veldt", why did the parents go back to sleep even though they knew the children disobeyed and broke into the nursery?

The fact that the children breaking into the nursery goes unpunished and without interference is almost lost amid the flow of the plot, because it isn't really addressed. However, it establishes some important characterization, particularly for Mr. and Mrs. Hadley. While it isn't explicitly stated, we are meant to understand that the Hadleys are not really parents in the behavioral sense of the word. They have lost, or never had, the ability to say "no"...

The fact that the children breaking into the nursery goes unpunished and without interference is almost lost amid the flow of the plot, because it isn't really addressed. However, it establishes some important characterization, particularly for Mr. and Mrs. Hadley. While it isn't explicitly stated, we are meant to understand that the Hadleys are not really parents in the behavioral sense of the word. They have lost, or never had, the ability to say "no" to their children, and they exist in the house simply as larger, co-habiting humans that the children observe ritualistic fealty towards. The children know their parents have no intention or ability to punish them, and so efforts such as locking the door simply embolden the children each time they defy their parent's authority. 

One important aspect of this relationship is the fact that the parents have, apparently, relied too heavily on the idea of an inherent, unspoken agreement between the children and parents, such as any family might have; that children obey the parents. However, the children, through the nursery, are able to circumvent their parent by establishing themselves as the movers and shapers; it is their fantasies that become reality, not the parents. The parents are reluctant to punish the children for a variety of reasons, but ultimately all this does is show that the children, via the nursery, have more physical influence in what will and will not happen. While the nursery is objectively a fantasy, to the children's undeveloped minds this is not the case, which explains their obsession with it, despite it appearing to be a simple entertainment device.