Yeah Chapman's times, even though it's a day walk book, are for a fit adult walking about 4 km/h, no lunch break or anything. I thought it might be only on mountain walking, and not coastal walking, but after doing his Narawntapu and Rocky Cape walk, the times are similar. With young kids I'd add 25/30% to his times. On the other hand, Tyrone Thomas' times are really on the safe side, with breaks etc... so if you've got his book, you can trust the walking times.
Regarding difficulty, a medium rated walk by Chapman would probably be rated difficult by Thomas. And a lot of long day walks (Mt Anne, Mt Jerusalem) in Chapman's book are presented as 2 day walks in Thomas'. Thomas also includes short walks (2 hours), which Chapman doesn't. It's more family friendly, but photos are seldom, and you've got to endure big chunks of text.
I would also like to add some classic traps that many people fall into when going to Tassie :
-Do not miss stuff like Cape Raoul or Cape Hauy just because most books say it's steep. It's not that hard, and definitely worth it.
-At Mt Field NP, many people talk about the falls and the forest, but to me it's all about the plateau. Unless it's winter and the road is awful, don't miss it. The real Mt Field is there.
-People talk about the Bay of Fires. But most people only go to Bay of Fires Conservation area, because that's the one signed for St Helens. It's good, but the real experience to me is to go to Mount William NP (which is the Northern part of the Bay of Fires). Roads are all unsealed, but the beaches are just magnificent, and the exploration very rewarding (hidden beaches and coves, beautiful campsites).
-Narawntapu is a haven for wildlife, but many people miss it because it's small and not that famous. Although the beaches are nice, it's all about the wildlife there.
-Cataract Gorge sucks.
-Accommodation is quite cheap in Tassie, have a look at
http://www.stayz.com.au/ . In some places, you can find a whole house with 2 bedrooms or more for 100 $ a night... Much better than a motel.