Have they sign an agreement which states both that any illustrations they do for the book or project in question is a "work made for hire" and so the rights belong to you; and as a back-up, that "should any of the Work be deemed to not be work made for hire, the ilustrator hereby assigns any and all rights to [INSERT YOUR OR YOUR BUSINESS'S NAME] and will execute any forms required for such assignment." Basically, have them contractually agree that the rights are yours.