Burn down chart is a decision tool

It is not a tracking tool. It tracks the estimated remaining work. It tells "where" you are, if your Sprint is still on track or not.

• Have Backlog Items added, which we put an estimated complexity number on (in relation to other backlog items).
• Before each sprint, we go through the backlog items in priority order (prioritized by the product owner), break them down into tasks for which we make a time estimate (in hours).
• When the number of available hours in the sprint are used up, the sprint is full

The team owns the estimates, nobody else, and it is the job of the ScrumMaster to guaranty that this principle is applied.

A Sprint Backlog and a Burndown chart are decision tools and should thus be representative of where you really are.

Make better estimates
If the team discovers a task that has to be done but that wasn't identified, the team must add this task and its estimate to the Sprint Backlog. And being not accurate in the start is not a problem, as long as you update the backlog with the knowledge gathered over time. The sooner you make these updates, the sooner you'll be able to adapt and take decisions.

It can be useful to keep the "initial estimate" and to compare it to the "actual time spent to complete". But not for tracking purpose, only to help the team to make better estimates.