(Exception regardless of decision for a d’var halocho, please?)
Source used: Children in Halacha by Rabbi Simcha Bunim Cohen.
Chinuch for not doing aveiros begins at an earlier
age than chinuch for doing mitzvos does.
The stage of chinuch in negative mitzvos is reached when
the child can be taught that something is forbidden,
that is, they will not merely stop now only to do it
again later (Mishna Berura, siman 343, se’if koton 3, b’shem “Acharonim”).
Rabbi Cohen says the stage is generally reached
at approximately four years of age (it varies).
(Thus far from CiH pages 7-8.)
According to this, you and your friend are not obligated
to teach your sons about muktzeh. (Whether it is a good idea
to start teaching muktzeh before you have an obligation to
is a question which it is not for me to answer.)
In addition to the requirement of chinuch, there is another Shabbos law regarding children. The Torah states (Shemos 20:10):
[v’gomer] ?? ???? ?? ????? ??? ???? ????, Do not perform any labor, neither you nor your son nor your daughter…
We learn from this that parents must not allow their children
to perform any melachah (forbidden act) on Shabbos. If a child does a melachah because he realizes that it will be beneficial to the parent, even if the parent does not instruct the child to do it, the parent is obligated to stop the child from doing so.
Since this Torah prohibition, unrelated to the law of chinuch, is part of the parent’s own Shabbos observance, it applies even to children who are below chinuch age.
Thus, the issue of chinuch and its appropriate age is relevant in regard to Shabbos prohibitions only when a child wishes to do a forbidden act for his (or her) own benefit. A parent whose child wishes to perform labor for the parent’s benefit must prevent the child from doing so, regardless of the child’s age.
[…]
It should be noted that if a child did melachah for his parents or strangers, they are forbidden to derive benefit from that melachah (pages 66-7, emphasis added).
I’m tired of typing this up, so for permissible ways to have a child below the age of chinuch perform melacha, see the rest of Chapter 12.
(You are never allowed to let him knowingly
do something that benefits you, though.)