Issue 5:
I've never watched KND, but I'm certainly familiar with Penny Gadget, one of cartoondom's great bondage queens. I like that you use that to such great effect. Also, Kuki putting her rainbow panties in Penny's pocket near the end was cute, sweet, and erotic at the same time.

Issue 6:
I was, of course, very glad to see a chapter involving a Phineas and Ferb character. And it was interesting that you actually set the events of the chapter during an episode of the show.
Having the girl crawl into Candace's Ducky Momo costume was clever. There's plenty of space in there, and plenty of privacy.
The Creamsicle bit didn't quite work for me. Isn't Candace's top red, not orange?
I'm not very familiar with Trixie Tang, but your mentioning that she's considerably younger than Candace added to the hotness of this story. I also wonder whether you chose her because of her last name. :)
"My cherry is so busted!" *snicker*
The atomic gumball machine sounds like a perfect P&F project.
This is the first chapter in which the sexual encounter isn't just a one-time thing, which I liked. The idea of Candace regularly going to Trixie's house to "babysit" her (ie, have crazy lezzie sex with her the moment Mom and Dad step out the door) is ever so hot. It also makes me wonder whether Candace would ever invite Jeremy along.

Issue 7:
I've never watched Grojband, but my wife was into Kick Buttowski for a while, so I'm slightly familiar with Kendall. In any case, it seems like this chapter is all about the meeting of two young lesbiatches.
I need to know: Is the Juicy Peach festival actually a thing on Grojband or Kick Buttowski, or did you make that up?
I like the bit about Trina's pussy grabbing Kendall's fingers so hard that Kendall could barely move them.
The jealous toady running away in tears seemed very apropos.
Did the part at the end about the parents not letting the kid have a band anymore actually happen?

And one more, general question: Are you planning any more issues?

This whole concept -- loli lesbian encounters described in a “Penthouse Letters” format -- is a good one. Not only does it allow you to write a lot of self-contained stories in an anthology format, but it helps the reader to suspend disbelief, because with the Penthouse Letters, I always had to wonder whether they were real or made up. (Of course, maybe if I had read them in my adult years rather than as a preteen who found the mags under his older brother’s bed, the answer would be more obvious.) Also, telling these stories in the voices of the characters is a bold endeavor; writing dialogue in a fandom character's voice is one thing, but writing the entire story in that character's voice is at a higher level of difficuty. 

Now, on to the chapters I've read so far.

Issue 1: 
--First off, I’m very happy that this series begins with two characters I’m familiar with. 
--Mabel’s intro sentence is nice way of starting off with some humor and with a solid portrayal of the character.
--The bus “accident” didn’t seem necessary (although it was very in character for Grunkle Stan). Save for Lisa, the Simpsons seem like exactly the sort of rubes whom the Mystery Shack was built to attract.
--The girl-on-girl action was fun. I particularly liked how much attention Lisa gave to Mabel's chest.
--The bit about the Mystery Shack having secret doors around the place -- and Mabel making use of one of them -- was clever.

Issue 2:
--I've never watched The Weekenders, but I'm guessing that Lor is a rough-and-tumble sort of girl, which made for a good contrast with Dee Dee.
--There was something really sexy about Dee Dee saying "baby hole."
--"Ooooh, what does this button do?" Heh.

Issue 3:
--I'm not familiar with either of the characters in this chapter. Rachel sounds really cute-sexy, though.
--I liked the Star Trek IV reference ("No, I'm from Iowa; I just work in outer space").
--I chuckled when the girl gives Rachel a play bite on her butt cheek and makes her yelp.

Issue 4:
--I'm not familiar with either of the characters in this chapter. Nonetheless, I liked the contrast between primitive and advanced. 
--It was cool that Betty's technological advantage and Fang's martial skills put them on a more or less even footing during their fight at the beginning.
--Even though it wasn't a big part of the action, I liked the finger-in-the-butthole bit because it distinguished this sex scene from the ones in the previous chapters.

Looking forward to reading more!