Sunstream is already cocooned before Strongbow and Moonshade recognize the third time. And Skywise says it's "three turns of the season" since he is cocooned at the beginning of The Discovery.
Recognition, two years pregnancy, Chitter growing into a very active toddler - all within three years. Elf children develop fast. Chitter must be a very early bloomer nevertheless.
Or - another point that was mentioned at times: Elvin gestation is "twice the time a human child takes". This would make it 18 months of pregnancy ... a bit more time for Chitter to grow.