Assuming that you are to write a different conclusion from the one that Ray Bradbury provides in his short story, you will want to be sure that your conclusion has verisimilitude; that is, your conclusion will need to be in line with the sequence of events in the plot. And, you will want it to be cleverly written and not just a pedestrian conclusion that describes the lions attacking Mr. and Mrs. Hadley.
Perhaps, then, you can somewhat imitate the direction that Bradbury takes his readers in his conclusion that has a break between it and the rest of the narrative. For instance, David McClean is due to return in half an hour to help the Hadley get to the airport, so you will need to have him arrive as does Bradbury. However, you can have something different occur when the two children call their parents to the nursery. For instance, maybe the parents have picked up something of Peter's that he has left in the kitchen and they carry it with them when they answer the distress calls of the children--"Daddy, Mommy, come quick--quick!"
So, instead of the just the parents being killed, Peter is also mauled by one of the lions who associates the new article brought into the veldt as belonging to him, making Peter a part of the "hated." Then, when David McClean comes over, he finds Wendy crying instead of having a picnic lunch in the center of the open glade. McClean can ask her why she is crying and she sniffles, "It wasn't supposed to be like this!" or something that will lead to McClean's perplexity about what has happened. Add whatever details you want, then. Maybe the nursery doorway closes suddenly at the end. (Hopes this helps to spur your own ideas. Refer to the links below to help you, also)