Is it permissible to imitate children's voices in self-talk without uttering it aloud?

A person is not held accountable for inner thoughts, passing thoughts, ideas, and imaginations, based on the Prophet's (peace be upon him) saying: "Indeed, Allah has pardoned for my Ummah what they whisper to themselves, so long as they do not act upon it or speak of it." However, one should guard the heart from these imaginations and occupy it with what benefits in matters of religion and this world, because giving in to them may lead to doing what displeases Allah the Almighty.