Ahh they started young, didn't they? :P
I love the ease of the relationship between Lily and Severus, and this is how they would have been in those early days. It makes me sad to remember the dissolving of their relationship, but for now, I think you've done a great job of writing their relationship. It's natural and I believe that they've been friends for years. James and Sirius are their perfect, cheeky selves. I really enjoyed this!