Not the parent, but for me I have to actively remember what the target voice sounds like and consciously alter my internal monologue to match. As soon as I relax this acting process my internal monologue slips back into my neutral voice. So those, "you're now reading this in X's voice," never really do anything for me.
Maybe related, I don't always experience my thoughts via an internal monologue. Maybe roughly 70% of my internal thoughts are abstract and nonverbal.
Can you remember someone saying something they said, in their voice?
Start with the memory, and then tack on the new sentence at the end of it.
Search your feelings, you know it to be true. Now you read the rest of this sentence in Darth Vader's voice.