[1] Oh hello, Peter! You figured it's time to grow up?
so, here my feedback is arbitrary- because it could be just representing how people talk, without concern for convention-, but i think "oh hello" without a comma in-between must be poor grammar, as evidenced by this famous clip in the notoriously bad "Tommy Wiseau" film "The room":
https://www.youtube.com/watch?v=zLhoDB-ORLQ
he says "oh hi mark", and the lack of comma after "oh" feels off, since "oh" is conventionally meant to signify that you're noticing something, which conventionally demands a pause, from processing the information. but i understand that there is also conceivable negative incentive to put a comma after "oh", because it comes across as disappointment. but that can be mitigated by the speech following. but- again-, sometimes speech really does just doesn't demand a pause after an "oh", such as in the phrase "oh don't worry", so it just demands on the context and what you're trying to convey. but the way you used it here does raise an eyebrow for me, from my thinking that it might be more natural for her to have a pause after the "oh"
[2] Don't call me Mrs. Bell, friends call me Tanya. My tomboy of a daughter told me all about you!
so, socially- in informal writing-, people are very insecure about using semicolons, because it's seen as pretentious. even i myself, i've been praised for not using semicolons before, when objectively my formatting demanded it, but i refrained from using them because they're socially unacceptable. but if you don't mind using semicolons, i think replacing the first comma ("Mrs. Bell, friends") with one might work better, since semicolons indicate a division in a sentence, which is read as a longer pause than a comma
[3] I find Lost Boys very fascinating, we should all play together some time! After they're done playing with that skank Wendy, I mean
the first comma here ("fascinating, we") could just be a period, since even making it a semicolon would be unnecessary, since even though the latter comment is inspired by the former, not everything needs to be separated by a semicolon
[4] Did you bring some magic dust? I could really use some right now, sweetie. If you are willing to share, auntie Tanya would fly you to the ends of the earth!
this is fine, in my opinion
[5] That's the stuff... Then, regarding your wish... Oh, don't say it out loud, it's bad luck! First have a look, maybe you'll see anything you like
the second-last ("loud, it's") and last ("look, maybe") comma here could be semicolons, but- again-, i am pretty insecure about using them (in informal speech, anyway) myself, so if it were me i might just restructure the sentences entirely to avoid feeling that semicolons would be necessary to do them justice. of course, you don't have to force characters to speak in a way that doesn't reflect them properly for the sake of meta (outside of world you're writing about itself) reasoning like this, but, it's something you could consider, if you were insecure like me, i guess. also, "anything" should be "something", and "you" should be "you'd"/"you would", since the "you'll" just before establishes the sentence as being future tense, so doing this will make it match the tense
[6] Do you think I'm a bit big for a fairy? You are big for a boy yourself <3 It's not the first time I'm playing mommy, you see. I've been the spirit of fertility long before I started granting wishes
it's personal preference, but i think the heart being an emoji makes it difficult to pause on it as if it were a period. i understand, though, that were it an actual conventional heart in any form, no one would put a period after it, whether it be a caption, or even informal texts in real life (part of that might be due to fear for punctuation breaking the emoji and turning it into its colon command (e.g. ":heart:", but still). i personally steer clear of stuff like sound effects and hearts interrupting speech, so i can't tell you how to properly format this, sorry. but i do think the heart being just an emoticon makes it not hit as heard as possibly even intended
[7] Oh come on, run your hand between my thighs already. Can you feel the fairy nectar running down, staining my stockings? That's the magic kicking in. You can put a few fingers in... Screw that, drop your pants, I want to choke on your dick right now!
i think the last comma ("pants, I") should be a semicolon, but i say this with the same social connotations asterisk when it comes to semicolon use i've been elaborating on earlier
[8] Gwah <3 ~ Don't be shy, I can't say no to the taste of boys' cum and we are far from done. Get on your back and take a close look at auntie Tanya's bald hungry snatch twitching in anticipation. Once I lower these swolen lips on your shaft, there will be no going back <3
i think the first period ("shy, I") should be an ellipsis ("..."). making it a comma doesn't have a long enough pause in my opinion, and making it a period would make it demanding, rather than accepting. i would add a comma in-between "cum and", since the added delay would provide opportunity to make the part before the added comma to be soaked-in by the reader. the following is arbitrary, but i personally would use "hairless" (or maybe there are other options) rather than "bald", because "bald" evokes imagery of men suffering from male pattern baldness, which in my opinion breaks immersion at best, and ruins the mood at worst. "swolen" is misspelled; it should be "swollen", with two "L"s
[9] Ah <3 So, the boy is no more... Can you feel auntie's pussy melting your dick? My knees are weak, so now it's up to you to bury your hands in my funbags and plow me like a washed-up junkie whore I am <3
the "a" in "a washed-up junkie whore" should be "the" instead, since she is specifying that she is referring to herself, rather than one of many
[10] Did you get a little carried away? I can still feel your thick seed draining into me... I hope my boy isn't tired of playing father yet <3
this is fine too, in my opinion