Do let coworkers know ahead of time that you'll be bringing your child. You might send an e-mail the day before.
Don't bring your child if your office needs to convey a professional impression (if clients are in and out of the office all day, for instance).

Do pick a time when the office is less busy, such as lunch hour, for introductions. Ask co-workers whether they're free before entering their work space.
Don't ask colleagues to look after your child while you attend a meeting or do other tasks; it's not their job.

Do have activities for him so he won't draw on your papers or, worse, someone else's. In a pinch, sticky notes, a few envelopes, and junk mail can keep him busy.
Don't allow your child to play with equipment like copiers and faxes ("Guess what, Mommy? I called China!"). Explain as soon as he gets there what's off-limits.

Do limit your child's stay to a few hours.
Don't bring him in for too long, or he may not be able to behave and will be more likely to get in people's way.

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