An absolute phrase is a phrase that modifies a noun in a sentence, but it is not connected to the sentence by a conjunction. It is set off with a comma only, and it could be removed from the sentence without changing the meaning of the sentence.
In this case, "His chubby legs pumping, the baby toddled across the room with his first wobbly steps." His chubby legs pumping should be the absolute phrase in my opinion.
Hope this helps!