Re: How fast is the kid going?
I like what you've done here, and i think it looks good.
As to your question for what else could be done:
Personally, i would soften your mask a little bit. The edges of the "thing"(?) he's sitting in, while sharp, still have a little dirt in them. As for the blur, i would select the child AND the (device? hahaa) and add a radial blur to them, that is centered on his face. This radial blur wouldn't have to be nearly as large as the one you added to the background, but instead would ideally make the outer edges appear as the feet do, blurred in motion. However, his face would still be non-blurred and sharp.
You might play with motion blur as well, as if the axis of the spinning is from where the shot was taken, instead of under the boys seat.