From what I can understand, the reason "les" is being used is because "them" is a direct object. Leur is used when it "them" is an indirect object.
E.g. - "Elle les aime" - She loves them. The object receiving the action of the verb is "them" so it is "les"
"Elle leur donne un livre" - She gives a book to them. The object that is being given is "un livre", which makes it the direct object. "Them" is the indirect object because they are getting the direct object, making it "leur".