What should my sister in law do if she gave her child too much phenergan (promethazine)? She gave a 3 year old 25lb child a 12.5mg suppository instead of cutting it in half. What should she watch for or do?

Monitor closely. Over a period of 4-8 hours you might notice excessive sedation & slow breathing.It's hard to say when peak level is achieved as rectal meds can have irregular absorption rates. As long as the kid is pink & arouses easily i would just let him/her sleep it off.