That is VERY common (of course,) as kids have the habit of growing up.
If the ticket has been partially used (as in, it has what's left of a non-expiring ticket) all you have to do is take that child's ticket, AND the (former) child to any ticket booth or guest relations and the CM will issue a new "adult" ticket with the same assets… all at NO CHARGE.
If the child's ticket has never been used…
THAT is a different story.
Take it to any ticket booth or guest relations and the CM will issue a new "adult" ticket with the same assets…
but you must pay the difference between what you originally paid for that child's ticket and the current cost
of an adult ticket with the the same assets.