Nimue had been making preparations. She'd prepared a second bedroom and furnished it with clothing and other necessities for a boy his age. As she worked, she'd been trying to devise a plan to retrieve Mordred. She was not sure of his relationship with this family, and didn't know how gentle he would want her to be with them. Killing them outright seemed a bit extreme, even for her. Simply taking Mordred would result in them thinking he'd been kidnapped, and starting a manhunt that would be an irritation, at the very least. Merlin would no doubt try to talk sense into them, or stage some sort of deception that left them feeling that they'd done the right thing.
The simplest thing, Nimue decided, would be erasing their memories of the boy. He was not their son; their memories were false to begin with. She didn't share her plan with Merlin, thinking he would likely disapprove of mind-altering magic. Memory wipes were not simple spells, so she planned to slip in and cast the spells while they slept.
Then her planning was interrupted by Mordred's call for help. Teleportation was a great deal easier when she knew just where she was going, and while she had attuned herself to Merlin, teleporting to Mordred took more concentration. It was urgent, though, she knew, so she was there beside him before he even finished his message.
Nimue placed a hand on Mordred's back while her eyes swept over the scene, taking it all in quickly. Dead intruders, frightened parents brandishing weapons. They'd obviously retrieved them from the would-be burglars, and Nimue doubted they knew how to use them. From the way their hands trembled, she was relatively certain they would not use them against Mordred.