This isn't slash as far as I remember. But I found it very cute.
This is what I remember:
Spike and xander have both been turned into children. The AI gang are looking after them. I think the story consisted of several small ficlets that each showed a glimpse of their life as children. One involved Xander going to the beach with Cordelia while Spike had to stay at home due to him still being a vampire. To comfort him Angel transforms the basement into a beach. Another involved Spike telling off the mall santa for making Xander cry.
Hope this triggers something in someone. Also I think the story was written by to separate authors.