Any advice on getting a 3 yr old w/a canker sore on tongue to eat/drink?

My 3 yr old granddaughter has a canker sore on her tongue. Also has slight tonsilitis. The Dr gave her Nyastatin for canker sore and Amoxil for tonsilitis but she is still complaining of pain. Giving Motrin also. She wont eat/drink very much at all saying it hurts. Realize she needs nourishment. Am very worried. Any advice?