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I recently came across an example of someone saying 'お返しする' rather than 'お返しします' about 18 seconds into this video:
https://www.youtube.com/watch?v=vazHI4jN6lc&t=18s
Under what circumstances should people use the short form of an extra modest expression? Or does it have something to do with the fact that the speaker is a child?
A simple answer is, yes it is used as お返しする since the speaker is a child. It looks like the girl speaks without です/ます endings, and that is why お返しする is used.
Probably a confusing thing is that お返し is certainly お＋返し, but to mean the favor in return, お返し is the only possible form.
You can see many samples mentioned in the definition of 返し are prefixed by お, and those お cannot be omitted.
In other words, お in お返しする exists because of vocabulary, and not (very much) of 敬語. I cannot really think of other phrases (usable by a child) to mean return the favor than お返しする. (Maybe お礼する is possible, but again お cannot be omitted)
