My 10-year-old daughter won a raffle in her class for a very good quality metal thermos. There is a Keilim Mikveh right next door to her school building and she would like to know if she can Toivel it herself, or if she must give it to an adult to do for her.
Technically, she may immerse it herself, however, it should only be immersed in the presence of an adult which is defined as a boy or girl who is above the age of Bar or Bas mitzvah. In such a case, the adult supervisor is to inform you that the vessel was immersed properly by your child. The same would apply if your child gives the vessel to the adult for the adult to immerse, that the adult must inform you that they immersed it. If your child is the one doing the immersing, which he may, then he is to recite a blessing over the immersion. If your child claims that he immersed it properly but there is no adult to testify that he did so, then you should re-immerse it without a blessing, if you or anyone else desires to use the thermos. However, your child may personally continue to use it on the basis of his claim that he immersed it. He may continue using the thermos even after he becomes Bar/Bas Mitzvah, on the basis of his acclaimed immersion while he was a child.
Explanation: Technically, if a vessel was immersed in a Mikveh by a child then it is valid, as the Torah did not require for it to be immersed with any intent of Lishma, and it is hence unlike the invalidation of a child to make Tzitzis or Tefillin due to them not being a Bar Daas who can do so Lishma. Nonetheless, it is debated amongst the Poskim as to whether one may trust a child [who is below Bar/Bas Mitzvah] who claims to have immersed a vessel in the Mikveh. Ideally, by biblical commands we do not believe a child testimony, and hence the above debate of whether a child testimony of immersion may be believed is dependent on the question of whether the mitzvah to immerse a vessel’s is of Biblical or Rabbinical origin. Practically, we rule that a child is not believed to have immersed the vessel if it required biblical immersion [i.e. metal] unless an adult who was there can testify to having supervised the child do the immersion. Based on all this, it seems clear that in our case in which we are dealing with a metal thermos the child cannot be trusted to immerse the thermos by himself and it must be done with an adult present who can then testify to you that it was properly immersed. However, in truth in the case above the matter is not so simple, as a) the thermos belongs to the child and why should the child not be believed about his own immersion if he knows for a fact that he immersed it. [In other words, perhaps the entire invalidation of trusting a child to immerse a vessel is only when the vessel belongs to an adult, however, when the vessel belongs to that actual child, then certainly the child may trust himself to have done it.] b) perhaps since the vessel belongs to the child it is not even obligated in immersion being that a child is not yet obligated in Mitzvos and hence his vessels would at most only be obligated to be immersed due to the mitzvah of Chinuch which is rabbinical and for which a child may be trusted. So in truth, seemingly certainly the child may trust himself to have immersed it, and the parent may allow the child to use the vessel if he claims to have immersed it properly. Furthermore, he may even continue using it after he becomes bar/bas mitzvah on the basis of his knowledge that he immersed that when he was a child. We find a similar ruling regarding a child who tied Tzitzis to his Tallis Katan, in which some Poskim rule that he may continue wearing it as an adult if he claims to have done it Lishma. A similar ruling is also found regarding allowing a child to eat meat from an animal that he personally slaughtered in an acclaimed valid method. Nonetheless, we did not conclude with this option of allowing the child to immerse their thermos being that a) in truth the vessel belongs to the parents, as anything a child acquires Halachically belongs to his parents being that the child is supported by them and is part of their household, and they are hence Zocheh [hold the legal ownership rights] to his findings [Metzia] and income earned. Accordingly, since it is possible that adults will come to use the thermos, and an adult may not rely on the testimony of the child, therefore it is proper for to be immersed in a way that anyone can use it, and hence have it immersed under the supervision of an adult, and have that adult inform the parent of is proper immersion. [Accordingly, even though the found objects and income of a child only belong to his parents on a rabbinical level, while biblically they are truly the property of the child, and hence once again we can argue that the mitzvah to immerse this vessel which was given to the child is only rabbinical, for which he can be trusted, nonetheless, since the vessel will require biblical immersion when he becomes an adult, and its immersion as a child cannot be accepted by adults, therefore, it is best to immerse it under adult supervision already now so it does not become a problem later.]
Sources: See regarding that a child may not be trusted to immerse a vessel: Michaber Yoreh Deah 120:14; Terumos Hadeshen 257; Taz 120:16; Darkei Teshuvah 120:105; Kitzur SHU”A 37:12; Admur 432:10; 437:6 regarding trusting a child to perform a Rabbinical Bedikas Chametz See regarding that the child may personally use the vessel on the basis of his own immersion both as a child and afterwards when he becomes an adult: Habayis Bekashrusos p. 164 footnote 74; Tevilas Keilim Kehilchasa 16:7 footnote 13; Chanoch Lanaar 41 footnote 6; Pesakim Uteshuvos 120 footnote 453See regarding that even after bar mitzvah a child may continue to wear Tzitzis that he tied as a child: Biur Halacha 14:1 “Lehatzrich”; Piskeiy Teshuvos 14:3; Mishneh Halachos 3:229; See regarding that a child may eat meat from an animal that he personally slaughtered in an acclaimed valid method: Darkei Teshuvah 28:116; See regarding if the vessel of a child is obligated in Tevilas Keilim: I have not found this matter discussed in the Poskim. VeHashem Yair Eiyneinu. See regarding parents acquiring children’s found objects and income: Admur Hefker Vehasagas Gevul 9 and O.C. 366:13; Michaber and Rama C.M. 270:2; Rambam Gezeila Veaveida 17:13; Mishneh Bava Metzia 12a;