Address Hagia Sophia, Istanbul Map,
Hagia Sophia was the largest church in the world until the Cathedral in Sevilla was built, and it's dome served as a model for many other buildings, including the Blue Mosque.
While you'll take Hagia Sophia at a faster pace with kids than you might otherwise, it can still be enjoyable. Ramps connect the lower and upper floor, and you should be able to push a jogging stroller right up the ramp. With a walking child, you might play these up as a secret passage to be explored (it worked for us).