A child under 10 cannot be guilty of a crime in Wisconsin
As Mark points out in the comments, the relevant statutes restrict the application of criminal law to children 10 years and older. For the rest of this answer, I'm going to assume
A child under 12 having such a weapon would be a Class A misdeamanor
Per Wisconsin Statutes 948.60(2)(a):
Any person under 18 years of age who possesses or goes armed with a dangerous weapon is guilty of a Class A misdemeanor.
However, there's also the aforementioned exception for long-barreled rifles and shotguns in 948.60(3)(c):
This section applies only to a person under 18 years of age who possesses or is armed with a rifle or a shotgun if the person is in violation of s. 941.28 or is not in compliance with ss. 29.304 and 29.593. This section applies only to an adult who transfers a firearm to a person under 18 years of age if the person under 18 years of age is not in compliance with ss. 29.304 and 29.593 or to an adult who is in violation of s. 941.28.
Section 941.28 refers to "Possession of short-barreled shotgun or short-barreled rifle," and is not violated, so this section wouldn't apply for that reason (this is why the underage possession charge was dropped against Rittenhouse). Section 29.593 simply describes the "Requirement for certificate of accomplishment to obtain hunting approval."
Section 29.304 covers "Restrictions on hunting and use of firearms by persons under 16 years of age," and is the most relevant section. Specifically, 29.304(1)(b) states that:
No person under 12 years of age may have in his or her possession or control any firearm unless he or she is enrolled in the course of instruction under the hunter education program and he or she is carrying the firearm in a case and unloaded to or from that class under the supervision of his or her parent or guardian, or by a person at least 18 years of age who is designated by the parent or guardian, or is handling or operating the firearm during that class under the supervision of an instructor.
So if the hypothetical armed child were 10 instead of 9, they could be charged (at least in juvenile court; I'm unclear on the rules for whether it could be charged in adult court) with that.
An adult who gave a weapon to a child (either 9 or 10 years old) in this manner would be guilty of a felony
Does this mean I could just buy a rifle and then give it to a random 9 year old? And if said 9 year old then went and hurt someone with it, there would be no consequences for me?
Per Wisconsin Statutes 948.60(2)(b) and (c),
(b) Except as provided in par. (c), any person who intentionally sells, loans or gives a dangerous weapon to a person under 18 years of age is guilty of a Class I felony.
(c) Whoever violates par. (b) is guilty of a Class H felony if the person under 18 years of age under par. (b) discharges the firearm and the discharge causes death to himself, herself or another.
So the adult could be charged with a felony, the class of which would be determined by whether anyone was killed due to the firearm being discharged. The same possible 948.60(3)(c) exception analysis applies to this as well, but because the person under 18 years of age in this scenario is not in compliance, the adult is also guilty.