Hmm... enjoying this a lot! The ushy gushy was yummy and Robin wondering what a kiss is like...lovely! I enjoyed the lines about how strangely domestic the scene was when Robin returned and you know, Robin is just as charming as a teenager as he is as a child.
Keep an eye on your tenses, occasionally (not frequent enough to be annoying, don't worry) you slip into present when it should be past tense. I'd suggest keeping it all in past tense, except for dialogue, which should mostly be in present. This is just a suggestion, and meant to be helpful. You're doing a good job and I'm definately enjoying the story.
One other little pet peeve of mine, which perhaps I shouldn't mention, that you've inadvertantly treaded on was Piggy calling Robin, "Robbie". Piggy has a slew of nicknames for Kermit, and has on one occasion called Rowlf, Rowlfie, but other than that she doesn't really tend to use diminutives for the other characters. I suppose, because she's now living with Robin, she might take it up... but it just feels weird to me. It's one of those gut reaction, nails on a chalkboard kind of reactions.
Still, that's just my opinion on the matter, and we all have different takes on characters, so by all means, if it feels right to you, go ahead and keep at it. It won't stop me from reading, or enjoying your writing.
Oh, and a few more specific things I liked, your use of the book "Oh, the places you'll go, and Valen as the name of the teddy bear, and Kermit's advice to Robin, all very sweet.
After about fifteen years of living with these two, it was still just a little weird, not to mention unusual, when he caught them in moments of domesticity like this. It was just so…unlike them. Normally, either Kermit was busy in his office writing a screenplay, or Piggy was at a photo shoot, or out shopping, or they were off at a failed attempt at a romantic evening alone, (because it’s impossible for them to have an entire evening alone) or clamoring for each other’s affection. However, to catch them in moments like these made Robin wonder, Am I in the right house? It just wasn’t natural.
I loved that whole paragraph! And Robin's right, it is weird, but sweet and lovely and good. "Clamoring", nice word choice there, and such a great image of Kermit and Piggy together. <is not a fangirl> <shifty eyes>
I'm looking forward to the next part! <hugs>
Leyla