Also, there may be a reason why your child does not like milk - lactose intolerance (unlikely) or simply taste or texture. You can discuss it with the pediatrician
My advice is to get it checked out with a healthcare professional.
Hope this answers your questions
Addendum: just looked up Avil and in Australia, Avil is a brand of pheniramine maleate, which is a first generation antihistamine. It can and might make your aunt sleepy. For other side effects, I would look on the package insert. Hope this information helps.