From: Rabbi Moshe Sofer
To: Rabbi Shmuel
The Chasam Sofer rules that a berachah must be recited at the tevilah of a minor convert. When the mother converts together with the child, the child cannot protest at all, removing any concern of berachah levatalah. Even in cases where protest is theoretically possible, the strong chazakah that minors do not protest renders the conversion a definite benefit, validating the berachah from the outset.
Source: Shu"t Chasam Sofer YD §253