Credit for the plot goes to thatCHILDbeME
Plot
Lillie winds up having a panic attack in the middle of the night from a rather terrifying nightmare that scares her to her core, however Donnie hears Lillie and decides upon himself to check up on his little sister and calm her down.

It was around 11:45 pm, rather late into the night. In the lair, Leo was fast asleep in his bed as he hugged a pillow while he had a sleep mask over his eyes for his 'beauty sleep' while Raph was hugging a few plushies in his sleep whilst he slept on his side with a small tired grumble and Mikey was sleeping peacefully in his hammock, one arm and one leg dangling out of the hammock as he didn't know it but he drooled a little on his pillow with a soft snore.

However one brother wasn't asleep, in fact he was wide awake in his lab, Donatello. The softshell was busy working in his lab and fixing up his spider shell after it had malfunctioned a few days ago, he had been more caught up with other projects that he never had much time to get around to repairing it. Donnie set aside one of his tools snd grabbed ahold of his purple coffee mug, drinking a bit of coffee which kept him awake from the caffeine buzz.

He wasn't aware that something else would happen tonight other than him working on repairing his Spider Shell. While Donnie was busy in his lab, there was one other member of the family who was fast asleep, Lillie. She was curled up in her bed and peacefully sleeping with her plush bunny in her arms, Lillie gave a quiet whimper as she squirmed in her sleep and she moved a little in her bed.

The poor kid was having a nightmare and it was a pretty bad one as well, Lillie squirmed more and she nuzzled her face into her pillow to try and hide but soon she woke up and sat up in alarm.

Lillie looked around and she panted shakily but she couldn't just forget the nightmare so easily, Lillie gripped her blanket and unknowingly everything around her began to overwhelm the girl which made Lillie start to hyperventilate.

She looked terrified of what was going on and she wasn't aware that she was having a panic attack, but the nightmare she had definitely caused this sort of panic attack to be induced.

Donnie kept working in his lab until he stopped and he arched a painted brow when he began to hear small pants or cries coming from the little sister's room, Donnie groaned quietly since he thought the kid had another bad dream so he went to take a look.

He got up from his desk and left the lab to Lillie's room as he thought to himself quietly, when he got there he moved the curtain door to Lillie's room. "Another bad dream L–"

Donnie stopped when he saw Lillie holding onto her blanket in one hand and her other hand was around her neck, worry laced his features as he went to his little sister and realized that she was having a panic attack.

"C..can't.. I can't.." Lillie choked out, she had a hard time breathing and catching her breath due to her panicked state as Donnie was in shock and hated that Lillie had to experience panic attacks like this at her age.

He tried to think of what to do since he had a fair share of panic attacks as well, Donnie remembered how Mikey would always let him know what he'd do whilst he was in his panicked state to make him feel more reassured so he took a leap out of his comfort zone to help Lillie.

"Lillie, I'm going to hug you.. is that okay?" Donnie asked, Lillie gave a small but light nod and she was gently pulled into a hug by the softshell as she began to slowly calm down but she still had to get her breathing under control. Donnie carefully but softly moved Lillie's hand away from her neck and Lillie looked at him with her bright orange eyes.

"You're okay Lillie, just follow my lead okay?" Donnie said, his tone a bit more gentle as he taught Lillie how to take breathing techniques for panic attacks. Lillie understood and followed Donnie's instructions as she copied what Donnie did. She had to breathe through her nose while she counted to four, holding it for exactly four more seconds and then exhale with the remaining four seconds.

Lillie caught her breath and her panic attack came down as she felt a lot better while Donnie rubbed small comforting circles on the kid's back, he was just relieved that Lillie was alright. "The nightmares got worse didn't they?" Donnie inquired.

"Yeah.." Lillie mumbled, she felt bad that she had woken Donnie up and she hid her face in his plastron as Donnie gave a soft hum but he figured that the kid needed all the comfort she could get. "Do you want me to stay here with you? Just until you fall asleep again." Donnie said, earning another nod from Lillie.

Donnie tucked Lillie back into bed but Lillie scooted over so Donnie could at least be comfortable too, Donnie laid on the bed and he kept Lillie company as the kid nuzzled up to him and seemed to feel a lot better. "Thanks Donnie.." Lillie mumbled.

"No probbles, don't worry I'm not going anywhere." Donnie said, he didn't know it but he was starting to feel sleepy too and he wound up dozing off thanks to the caffeine wearing off and Lillie fell asleep after a bit of time.

Donnie had a protective arm around Lillie to keep the kid safe as she slept and reassure Lillie that no more bad dreams would hurt her, he didn't know it but his brothers would do the same thing when they were tots whenever he had some unpleasant dreams.