If you go to Dubrovnik, and if you are there for anything more than half a day, you will likely walk around the city walls anyway, so in some ways, this journal is redundant.
Having said that and having recommended that you do take the walk, I would offer the following suggestions, which may well be worth considering:
The walls provide a great overview of the city, so if you are in Dubrovnik for a few days or longer, get up on the walls as early in your stay as you can.
The walk takes over an hour and a half, and more than two with children. We did the walk with our four-year-old daughter, though, so don't be put off by having young children. Take plenty of water up with you, and wear a good hat.
The walls are busy, and parts of them are steep, so take your time.
Most locals we spoke to recommended doing the walls in the morning, but given that they don't open until 10am, you are likely to still be up there at midday, so be aware of that.
Having said all that, nothing remains than to recommend that you not skip this attraction. The walls are kept in great condition and are very safe. The views are magnificent, and as with any city wall walks, there is a slightly ridiculous sense of achievement when you have completed the circuit.