Sketchpad and Sketchpad 3.5. I will be focusing on the online version Sketchpad for this post. Sketchpad 3.5 is the offline version and did not run as smoothly as Sketchpad.
For awhile there was a lack of vector image editing programs on Chrome OS, but recently several have solid ones have emerged. Sketchpad offers a variety of features and tools that would be attractive to elementary age students.
For example, under brushes you can choose a bear (fur) or a car (chrome) which both make for a nice effect. Under the stamp you could choose butterfly or leaf for a stamp or a brush effect.
Clipart is where you really get to see the strength of a vector image editing program. It does not matter if the clipart is icon sized or covers the whole screen, its clarity stays intact.
Lastly, where Sketchpad excels, for me, is in its export functionality. It allows you to save to your computer or SD card in png or jpg, or export to Google Drive itself.