Labor and labour are two spellings of the same noun. The spelling depends on the type of English.
In American English, you write “child labor.”
In British English, you write “child labour.”
The same difference applies to similar words, such as “behaviour or behavior,” “honor or honour,” “color or colour,” “favorite or favourite,” “favor or favour,” and “humor or humour.”
It’s important to choose one type of English and use it consistently. The QuillBot Grammar Checker can help you with this.