A Bavarian court accused a popular German folk singer of breaking its strict child labor rules for allowing his 4-year-old son to perform on stage with him during an open-air summer concert, The Associated Press reported Friday.

Angelo Kelly, 39, accrued a $3,600 penalty by the Hassfurt administrative court for his 2019 appearance at an evening concert, when he performed alongside his son, William, who sang “What a Wonderful World,” according to the AP.

William, the youngest of Kelly’s five children, stood on stage “for at least 30 minutes … where he played along, sang along and interpreted his own song,” the court ruled, according to the AP.

“This is considered work by the Youth Labor Protection Act,” said the court in the ruling. (RELATED: Girl Scout Cookies Tied To Child Labor In Indonesia, Malaysia: Report)

Kelly has stated his intent to appeal the verdict on his Facebook page.

“As a father, the well-being of my children is my most important! William was and is not obliged to perform with us at any time at our shows. If he did, it’s just because he wanted to,” said Kelly.

Under German law, children aged 3 to 6 can partake in musical performances for up to two hours a day but only between the hours of 8 a.m. and 5 p.m. and only with official approval of an exception, the AP reported.

Four-year-old William performed on stage until 8:20 p.m., breaching Germany’s child labor code, according to authorities.

Kelly’s lawyer, Julian Ackermann, criticized the ruling, saying that interpreting a “brief appearance on the stage in the presence of one’s mother and siblings at a concert given by one’s own father” as child labor was legally wrong and “far removed from the facts,” according to the AP.