The bathrooms on the ground floor (lowest level) of the Field Museum are excellent, particularly those by the Explorer Cafe. They have lots of space, tot toilets for little kids, and fold-down changing tables that aren't in a stall. When you have kids and time to get there, I highly recommend going out of your way to get to those bathrooms.
The rest of the bathrooms in the museum are pretty bad. They are tiny, though they do have nice changing tables. For whatever reason, the bathrooms on the upper level are often pretty unused, so that helps compensate a little. Both the main floor and upper level bathrooms do also have decent areas nearby to leave a stroller.
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