We have tours, guest speakers (i.e. the fire department) then they have relay races on how fast to replace batteries in a smoke detector. They visit their parents work area for about an hour, we have a mock clinic with displays and then we have a DJ. We also have educational computer games
At my previous employer there were fun areas where you learn about the company, company swimming pool, cafeteria, lab center, and maybe you can have your child help you out with "fun parts" of what you do perhaps. Meet & greet other employees.
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Next month our company is finally allowing us our first Bring Your Child to Work Day. A co-worker and I have volunteered to schedule activities for the day.