A Verb for a Child Conducting a Conversation between Toys

Solution 1:

Voiced can convey the idea of speaking for another.

Solution 2:

I suggest that a good word for your sentence is enacted.

Definitions of enact:

Macmillan Dictionary:

to perform a story or event as a play

American Heritage Dictionary:

To act (something) out, as on a stage:  enacted the role of Romeo.

… or perhaps playact:

Oxford English Dictionaries:

• Act in a play.
• Act (a scene, role, etc.).

Merriam-Webster:

• to take part in theatrical performances especially as a professional
• to make believe

Note: enact is listed by twice as many dictionaries as list playact.

Other words I looked at and dismissed include “dramatize”, “portray”, “simulate” and “depict”.  If you don’t like the two words I chose, maybe you’ll prefer one of these alternatives; or you can look them up in a thesaurus and get even more synonyms.