Question: I tried buying a passul set of arba minim for my children, but the shop wouldn’t sell them to me, claiming that they could only use a kosher set. If they can’t tell the difference, does it really matter?
Answer: The Mishna (Sukka 42a) teaches that children are obligated to shake lulav and esrog when they are old enough to do so. Therefore, the Shulchan Aruch (OC 657:1) writes that parents must buy a lulav and esrog for their child when they are old enough to shake them properly. The Mishna Berura (128:123) explains that this age varies between different children and mitzvos.
The Biur Halacha (657:1) stresses that children must only be given a kosher set that is fit for an adult to use.
The Shulchan Aruch (OC 658:6) writes that one should not give their arba minim to a child on the first day before fulfilling one’s own obligation, as while the child has the ability to receive a gift, they cannot gift it back. The Magen Avraham (658:8) writes that the child would not fulfil their obligation unless the arba minim belonged to them. The Mishna Berura (658:28) explains that the father would not have fulfilled his obligation in chinuch by lending his child a set. Nonetheless, he notes that other acharonim disagree, permitting lending one’s child a set. In Shaar Hatziyun (658:36) he clarifies that lending suffices (See Minchas Yitzchak 9:163:3.)
Likewise, R’ Shmuel Wosner (Shevet Halevi 8:152) demonstrates that a parent fulfils their obligation by lending them a set. As chinuch is only miderabanan, a child’s mitzva is like the second day of Yom Tov when one fulfils their obligation with a borrowed set (See Aruch Hashulchan OC 658:17).
Nonetheless, R’ Moshe Feinstein (Igros Moshe OC 3:95; YD 1:137; YD 2:104) writes that it depends on how we view the mitzvah of chinuch. If chinuch serves solely to prepare the child for when they are older, then a borrowed set is sufficient. If, however, the parents have an obligation to ensure that their children have performed the mitzva properly, then they need to own it. R’ Moshe concedes, following the Shaar Hatziyun, that children may, indeed, use a borrowed set, and parents would fulfil their obligation of chinuch. Children would not be able to recite the berachos, on such a set, however, unless they own it.
In conclusion, while it is ideal for parents to buy children their own set of arba minim, they must only use a set that is fully kosher.