Your Scouts may work on advancement with their parents as well as in your den activities. You may also assign them Do-at-Home Projects from time to time—items to finish at home and bring back to show the rest of the den. It may be convenient to record this information during this part of the meeting. An assistant den leader will work well for this duty as well.
You will need to have a method to record this advancement. The BSA sells colorful wall advancement charts that can be used to keep track of each of your Scouts’ adventures. Using colored pencils or a signature to designate a requirement as completed can work well with this type of chart. If your pack uses a computer-based tracker and you or an assistant are proficient in entering information on a laptop or other device, then it may work well for you to track advancement digitally. You can learn more about digital tracking with MyScouting on Scouting.org. There are also paper pages available from BSA and other sources to assemble a binder and mark off individual requirements.
However you decide to track advancement, it is an important part of your den leader job. Figure out a method that works for you, and go for it! If a Cub Scout has completed an advancement item with his family or at school, have him give a short report to the rest of the den, so they can see that all advancement does not have to happen in the den meetings.
This may also work well in the Gathering activity time if you have help and the activity will not be interrupted.