Acacia Heartstrings, thank you

Unfortunately it's not in any of the soundtrack. I guess they didn't consider it a long enough piece to include.
Chipster Roo, I found the part you described. I never put two and two together, but it could be a little theme for Primrose.
There's another nice bit of music that wasn't in the soundtrack. It's a calm playful song, and occurs a lot. (season 1, episode 3 at 03:23 for example.) I think a human whistled the song at one point too.