Don't know if you'll get this in time. The queue can be 3 hours long! There will be staff around to tell you how long your wait is going to be. The tour inside takes about an hour and a half (toilet visit before you start). I have to say with all the standing and walking I'm not sure you can really do the tower and the dungeon in one day. I'm quite a fan of the tower. There's a lot to see in there.
You can buy tickets in advance on the website and jump most of the queue. It takes a good 2 hours or so to go through it all and I am sure the kids will enjoy it, although personally I think the Tower is much more fun!