Welcome to the Forum. Sounds like your toads are very lucky to have your son as an owner! I have seen Peace Lilies on tanks before, so I can't see there being a problem. I have a friend who is a florist, so I will double check with her and get back to you. I use Peperomia in my Fire-Belly toad tank and my toads just love them-the plant stays short and has thick, broad leaves that are perfect for lounging on. The plants themselves are very low maintence. I will try and get back to you on the Peace lilies as soon as I can unless someone else here beats me to it!!