chat theres a big storm outside and im lowkey regressing so im gonna write agere headcanons for the bad sanses (plus some extras) being a caregiver for little reader (written for someone who ageres to about 5-6 years old)
♡ 》 Nightmare : Nightmare, while at first isn't exactly use to the concept of someone age regressing. Once he does learn the basic idea he will HOURS reading about it, if it means his partner is comfortable and safe. Dude studies almost everything he can about it, even having Killer help him with the 'interwebs' to learn how to help you. It is hard for him to read the emotions that resonaite around your soul when you are in a little headspace, it being a messy mixture of sadness, overwelmed, clinginess, and many others. Once he does pinpoint the headspace in your head he is quick to work, but silent about his plotting. He quietly pulls you to the side, away from the others and in a comfortable hide away he established as a 'calm down' corner for you. Weither this safe space be a crate, a closet, a hammoc, anything you need he would do his best to create it. He would go all out with this, being a guardian and demi-god he is not afraid of getting anything you need, weither it be childish stuffed animals or kid blankets. He would sit with you in this corner, offering you a few activities (not too many, but 2-3 so you still have options). He would be willing to read to you, play stuffies, anything to calm your state of mind because he knows exactly what it feels like, as he can feel your emotions, and it pains him for his soulmate to be feeling those negative emotions.
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♡ 》 Horror Sans : probably the second most likely to know what age regression is. I see his brother as a big age regressor, and while Horror doesn't know what it is by name, he is quick to go into protector mode the moment you show signs of regression. He insists on keeping you in his lap, or close enough so if anything happened he csn grab you. If you are a clingy regressor this is your guy, he is all for cuddles soft comforting words. Unlike Nightmare hes less quite about you regressing, and unless you bring it up if you are uncomfortable with that, he will continue doing so. He will loudly state that tou are not in a good headspace and needs to be alone, that way no one slips up and says anything you can't handle in the moment. He makes sure you have plenty if snacks as well as tea for you to drink, constantly reminding you snacks are okay and tea will help you calm down.
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♡ 》 Dust Sans : A very silent caregiver. I would say he knows ALOT bout age regression, I see him as someone who does tend to feel smaller especially when his hallucinations become horrible. He can tell the signs almost immediately, and had supplies in his room from moments where he was feeling small that you can use. He offers coloring books, scented plushies, stickers, and as much cuddles as you need, but he is not the best with words. He will put on a kids show he enjoys and tuck you into a well made nest, making sure you are warm and comfortable. Alot of what he knows about age regression is from Nightmare, as his older coping mechanism would drive him to self distruction, something that he would never want you to be driven too.
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♡ 》 Killer Sans : A member of the bad sanses that has his own issues that leads him to pet regress, being either a fox or cat. He does this with ONLY Nightmare but understands the signs of someone regressing so when he sees you showing said signs he gets a bit tense. Hes always playing off others emotions and making jokes, but seeing someone in a messy headspace that he has been in before makes him a wee bit uncomfortable. It's hard for him to show that comforting side to others, and he would rather find Nightmare or Horror to help you out, but he seems to be your only option! He tries to distract you from any negative emotions, offering a number of snacks, kid show ideas, and once he gets a bit more comfortable with letting loose he helps build a pillow fort with you. By the end of the night he would be purring fast asleep, your regression helping him also slowely pet regress as you both cuddle in the pillow fort.
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♡ 》 Cross Sans : Unless you are blunt with him, this guy will NOT realize you are regressing. He has had no exposure to the concept or caring for people in that type of way in general. You would have to set up a keyword or hand signal for him to know. Once he does realize the headspace you are in he is in protective guard dog mode, similar to Horror. Staying by your side, he makes sure you have everything you need. Comfy blanket, stuffies, earbuds if things get too loud, anything you ask he will do his best to get it for you. He is stiff despite trying to lean into your childish demener when you do regress, and it takes him a while to loosen up and be willing to watch any kid shows or media with you. Convince him hard enough and he would do voices while reading you your favorite story to bed.
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♡ 》 BONUS!!! Swap Sans : I see him as someone who regresses when everything is extreamly stressful (totally not projecting here) so he is quick to be protective and at your side. He is the type to set you in his lap and cuddle you, making sure you have everything you would ever want around you. Favorite snacks, plushies coloring books, warm blankets out of the drying, you name it. This dude plans a whole nest and just keeps you in it while making sure you are calming down and comfortable. He is the type to sit you down after you are feeling big and make sure he did everything correct, seeing what you would need next time, things he can improve on, ect.