She probably will. The conflicting posts you are reading are about people over the age of 10 trying to order from children's menus. A five year old is obviously not over age 10, no matter what kind of dining credits they have. Check with your server. If your server is confused by the request and by the use of adult credits for a five year old that is the only reason I can think of for problems.
You can either use her adult credits that you paid for to pay for her child's meal (probably not the best value) or you can pay cash for her child's meal and use the adult credits for her or for other adult party members at other meals.