Hi Lynda
I found this info for you:
Disney never penalize you for your children growing older. You can take your leftover child's pass along with your now too old child (the child must be present to do this) to any Guest Relations window or major park ticket booth. They will update that child's pass to an adult one with exactly the same leftover admissions. There is no charge for this.
Saying this I had a similar problem when we were there at Easter and I took my childs pass to Guest services and they told me just to keep using it, they also said that a childs age is never asked.