Wis. Stat. § 48.619: Definition.

In this subchapter, “child” means a person under 18 years of age. For purposes of the authority to provide care and maintenance for a child and of counting the number of children for whom a foster home or group home may provide care and maintenance, “child” also includes a person 18 years of age or over, but under 21 years of age, who resides in the foster home or group home under an order under s. 48.355, 48.357, 48.365, 938.355, 938.357, or 938.365 that terminates after the person attains 18 years of age, under a voluntary transition-to-independent-living agreement under s. 48.366 (3) or 938.366 (3), or under the placement and care responsibility of another state under 42 USC 675 (8) (B) (iv).