For the last fifteen years, Stiles' spark of magic has been healing the Nemeton and helping it regrow from the stump that it was. It's not fully grown yet, but it's getting there. Even though Ofelia's told constantly by her fathers not to go to the tree, because they both still consider it to be dangerous, Lia can't help but be drawn to it. (Derek taught Ofelia Spanish from birth so she could talk to Aunt Cora's mate, and Stiles kept with his mother's tradition to give their children difficult names.)
Neither Stiles nor Derek can really blame Ofelia for stumbling into the labyrinth via the Nemeton, because they both know what fairies are like. But that doesn't mean that Derek wasn't worried like crazy, because Lia is usually smart enough to leave a note before she goes exploring the forest, but this time she'd just run from the house (she doesn't like it when Stiles and Derek argue; neither do they, honestly, but they're not perfect and arguments are bound to happen, even if neither of them can really remember what they were arguing about anymore).
Stiles was kind of regretting teaching her magic, because Lia knew enough to cloak herself from her Papa's werewolf senses (even though much, much later Stiles would be so freaking proud of her, right now, not so much). In hindsight, Lia realised that cloaking her scent wasn't her smartest idea to date, but her fathers found her in the Nemeton eventually, and while she was worried that it was all just another illusion, another trick to try and make her spill innocent blood, she counted her fingers and knew that she was awake and safe again. Caught up in her Papa's embrace, Lia promised him that she wouldn't cloak her scent again, Derek calming her.
While Derek hugged her to him tightly, thankful that she was all right and alive, Stiles showed his relief, fear, and worry in a very different way. Ofelia hadn't realised how emotional her Da was until he completely obliterated the fairies that had tricked her into going into the Nemeton. She watched with wide eyes as they fell to the forest floor one by one, bodies blue as their life essence is snuffed out. Her Papa holds her Da back, hugging Lia in between them and she holds onto her Da just as tightly as her Papa is. Stiles trembles with the burst of power and tries to ground himself, needing the anchor to his family - his pack - even more than his wolf right then.
After Ofelia nicks her palm on one of the Nemeton's branches and offers her own blood in order to compensate for the fairy magic it's just lost, she feels more than knows that the Nemeton forgives them. When her Da's calm again, they return home together, and Ofelia looks over her shoulder to the Nemeton, knowing that no matter what creature comes for her next, she'll always be safe. Her Papa and Da will make sure of it.