We hit it lucky, judging by the tapes showing how long the lines can be, and had no wait at all to get through security into the square, nor into the cathedral itself. We did wait about 30 minutes in line for the dome, but most of it was shady, thank goodness.
With our kids - ages 5 and 8 - we dont get a chance to read all the plaques and quietly contemplate life, but we do get a chance to indulge in the fun side of life! So, of course, we did the dome walk!!
Well worth the investment to get the elevator to the initial, indoor part of the dome. It was high and IMPRESSIVE - thats the widest bit at the base of the dome. Kids went free; so it was just the cost of the 2 adults.
Then, with sprightly wee legs, we walked through the inner chambers of the dome and climbed to the tippy top, where you can do the full circumference of the highest point of the dome outdoors. Theres a VERY large fence encompassing the whole thing, and we always felt very safe. No chance for the wee ones to squeeze through. Kids enjoyed the "sideways" walking on the inner journey as we approached the curviest parts of the dome, and still talk about having to "walk sideways".