You have received lots of good advice but I'll add my 2 cents based on the ages of yr kids, and what i learned on our trip to pisa, which was riddled w a series of small mistakes at least for me. We did it as a day trip from florence, on a sunday afternoon, trying to still get reset from our 5pm arrival the day before. I am glad we went...and tickets in advance are imperative if you want to climb the tower.
We got off at central station, it is a serious hike from there to the tower. Enough so that my internal dialog for most of the sclep was "where the hell IS this thing?" I cant imagine doing that w four kids who might have opinions about the distance, so id absolutely recommend rosore station. Or add the bus if you use central station
It was crowded at the field of miracles, enuf so that when 2 of my adult companions stopped to take photos and the other 2 of us walked on not knowing they'd stopped, we could NOT find each other in the crowd. So, w a family of six, have the plan we did NOT have, like "we'll meet on east side of church." I had preordered tickets, my 3 friends did the tower (i do not do heights) i did the bapistry, and was happy w my choice. Afterwards i waited in the lobby of museo delle sinopie on the southwest edge of the complex. This i would absolutely recommend for yr young family.... there's an area of padded benches and a screen showing some antimated short movies about attempts to straighten the tower through the centuries. To my memory there were 3 or 4 movies of less than 5 minutes each. If they were in Italian they had subtitles, and were very engaging. I watched the whole series about 3 times waiting for my friends. This area is free. If id known how much time i would have, id have paid for the museum, and the restrooms therein. When my friends came back i heard that at the tower they had access to restrooms as well as a mandatory bag check before the tower. I have no idea if luggage check can be used by people not climbing tower. There is a cluster of resturants off the south east corner of the grounds, but we did not leave that direction . So, we headed back on the long walk to central station thinking we'd find someplace open where they could get a gelato and i could use the restroom. Most of what's. On that route is residential, and the few establishments were locked up tight on a Sunday afternoon, and it was a miserable walk all the way back to that Centrale station toilet. Bottom line, it should be a great stop for yr family but learn from my mistakes.