I agree with the poster saying go early as it actually is less crowded.
I disagree with the poster saying do a tour unless your children are very used to and comfortable in large crowds - I tried the tour on two separate occasions with two different children and we dropped out and made our own way both times - the tours are far too big, far too many people, far too difficult to hear and far too much like being in the middle of a zombie hord... For the same reason I only saw the crown jewels with one child, if there are big queues the whole experience of churning through is quite unpleasant.
I've taken each of my 3 children seperately, each at age 6 or 7 and each really loved the tower though.
We always read a children's book about it before we go, and they have an idea what they want to see and about the history.
I always let the child take the lead and go at their pace.
I spent eight hours there with my dc2! His choice. He sat on the floor and watched every video in the white tower, he looked at every piece of armour and weapon and tried every interactive exhibit and was fascinated by the money/ mint exhibit, unexpectedly. Another of my children was fascinated by the stories of things that had happened there more than the exhibits and we read everything.
I'd do some children's book reading first and let the children take the lead. If two parents are going I'd be prepared to split up once inside and go one with each child as they may well get a lot more out of it that way.
Have fun!