Children understand the Indirect Object Transformation as early as five years.
"George," on the other hand, is the object of the preposition "to" in the adverbial prepositional phrase in the sentence, "Mary brought Billy to George."
For verbs in English which describe a reasonably rapid transference (e.g., "brought"), of something ("Billy") away from the subject ( "Mary"), we can do and often do a grammatically contorted thing.
We remove the preposition ("to") and reposition the object of the preposition ("George") between the object and the verb, and say, "Mary brought George Billy."
Notice how the word "George" violates the proximal rule. Under normal circumstances "George" should be the object, but in this transformation, he is the adverbial indirect object. Children, by age 5 years can understand this transformation.