You'll see millions of dollars being printed during a tour of the BEP. The tour features the various steps of currency production, beginning with large, blank sheets of paper, and ending with wallet-ready bills.
I took a tour of this during a school trip a few years back and thoroughly enjoyed myself. I found it very neat to see how money was processed and printed. It was eye-opening and not what I had expected. I highly suggest that you stop in and take a look for yourself. It's a definite must see when you visit D.C.
We took 6 kids ages 4-1/2 to 9 on the tour and they all really enjoyed it. The older kids enjoyed it more, but even the younger ones were entertained.
First off, it is a bit of a hike from the Smithsonian Metro stop so be prepared and leave enough time to get there before your tour starts (we were practically running to get there!) The tour starts with a short video (which the kids didn't really pay much attention to). Then we were taken through a guided walkway tour of several of the areas where the bills (or notes as they call them) are printed. Our guide was very knowledgeable and some of the people working even waved and interacted with the kids. The gift shop was a bit chaotic and I'm sure there are money souvenirs sold at other places in DC, so it might be wise to just slip out the door afterward.
A fun tour and you can't beat the price - FREE!
Now where else can you see millions of dollars? The bureau of engraving and printing in DC prints various paper money. The tour is best for elementry and up (but you can take stroller in and park them). As with all DC government sites you will go through security on entry - but the staff were very helpful to our family with small children. Once inside you have a formal tour through the printing process - it is well laid out so even little ones will get a good view. Make sure you use the bathrooms before the tour starts - no potty breaks while on the tour! Best for elementry and up. Entry is free but timed tickets are issued - in summer these may sell out early (tickets are handed out from 8.00am) - I suggest you stop by the ticket booth the day before find out how early you need to be there. Note the ticket booth is on the opposite side of the building to the tour entrance. Easiest access is from the orange/blue lines at smithsonian station.