This tugs at my heartstrings in so many ways, and all of them beautiful. First, you did a lovely job on early-transformation Simon and little Marceline looks super adorable as she gives him her full attention. The blanket and teddy bear add to her cuteness. It's also a good bit of context with Mr. Petrikov sporting his own backpack for the trip, or perhaps journey together. We can only wonder how they met and where their destination is – or destinations are.
It's wonderful to think of poor Simon having enough of himself left to give comfort and affection to such a sweetie pie, even if she is kind'a supposed to be evil. Maybe this would explain why Ice King never tries to ask out Marceline even though she's also a queen and also much older, because he still remembers that he's so much older than her because he knew her when (but is always looking for "his princess").
The fire is wonderfully done, especially with the shadow play on the characters looking great. But the most heartwarming part is that for a time his need to be loved was fulfilled in some small part.
Oops! I see that this is a potential spoiler! Please don't reply with any details for a few days until I get a chance to visit someone with OnDemand and watch the ep. For now I'll just enjoy my interpretation.