Child Toiveling Keilim
If a child under the age of bar or bas mitzvah is performing the mitzvah of tevilas keilim, immersing vessels in a mikvah, the halachah regarding metal vessels is as follows: If the child immerses the vessels in the presence of an adult, the tevilah is valid, and the child can even make the brachah. However, one may not have a child immerse the vessels on his own, since the testimony of a minor, even of a precocious child, is halachically unacceptable. Therefore one should not trust the child’s word that they performed the tevilah correctly. Whereas when toiveling glass vessels, for which the obligation is only rabbinic, or plastic vessels—for those who are stringent and treat them as they do glass vessels—one is permitted to have a child toivel them, and we trust the child that they actually toiveled the vessels properly.
https://asktherav.com/article-203-using-a-child-for-tevilas-keilim/