We live in Hong Kong with our 4 & 6 year olds. My kids have been to the Big Buddah a few times, but never walked up the steps. It's still worth the trip.
When we bring visitors out there we make a whole day of Lantau -- we take the MTR to Tung Chung, then the cable car to Ngong Ping (make sure you get tickets for the "Crystal" cabin -- costs a little extra, but usually a shorter line and it's a glass bottom car -- our kids LOVED it!), walk through the Ngong Ping tourist trap of a village, to the Big Buddah and then back to the monestary (gorgeous buildings some of which are being restored or built, not sure which). You can then grab a bus or taxi from Ngong Ping to Tai O -- a great little fishing village. Taxis are really hard to get in Tai O, so when you arrive check the bus schedule going towards Mui Wo (I think it's bus #1 and 11) so you know how much time you have to spend in Tai O. Usually 90 minutes is plenty time to walk all around the village and do a little boat ride through the village which then goes out to open water looking for Hong Kong's famous pink dolphins. We've done the boat trip probably 5 times or more, but only saw a dolphin once -- so don't get the kids hopes up too much.
Once you get the bus from Tai O, get off at Lower Chung Sha Beach. There's a famous beach side restaurant called The Stoep (pronounced "Stoop"), but we prefer a new place that serves only salad, pasta and pizza -- I forget the name, but if you walk past The Stoep and all the other restaurants, it's the last one. Amazingly fresh and delicious food. You can hang out on the deck or at one of the tables in the sand, have a drink and let the kids play around in the sand. Once you finish up there ask the restaurant to call you a taxi or go back up to the main road and take the bus to Mui Wo to catch the ferry back to Hong Kong. Here's a link to the ferry schedule. Try to get the fast ferry back, it's worth the price difference. http://www.nwff.com.hk/eng/fare_table/central-mui_wo/
As for other temples, my kids weren't at all impressed with Man Mo, but we've been a few times to the 10,000 Buddahs in Shatin and they love it each time because of all the different statues. We usually drive ourselves out there, but I bet there are buses available or the MTR goes to Shatin, although not sure how close it is to the temple. Here's a link: http://www.hongkongextras.com/_ten_thousand_buddhas_monastery.html
There are alot of steps to walk up, but it's a more gradual asecent compared to the Big Buddah, plus the kids are entertained by all the different faces of the buddahs along the path. And we've seen wild monkeys there, so keep an eye out (and don't bring food because they can be v. aggressive if they like what you have!)
You mentioned Duk Ling. We've not done that one, but there's another traditional junk called the Aqua Luna which we've done a few times. One was a half day trip which picked up in Central, went around the island to Stanley where we had time for lunch and then went back to Central. If you want to watch the light show, our favorite place to view it is from the Aqua Luna. It's the only option I know of being on the water to see both sides.
We were just at Top Deck on Sunday. You might want to call ahead and ask if they have a kids menu if that's all that your kids eat. I didn't see anything on the buffet line that was aimed just at kids. Mine loaded up on mussels, sushi, basmati, ribs, crab legs and corn on the cob. Just a warning about the "play area for kids" that they advertise -- it's a tiny room with padded floors, a puzzle or two and coloring books. Although they do have a sitter in there, your kids might be bored with it as mine were. You mentioned taking a sampan there. If you do that you might want to start from the Aberdeen Promenade because its a longer boat ride and the driver can take you through the fishing boats first then drop you at the Jumbo. Otherwise you can get a free shuttle boat (owned by the Jumbo and not a sampan) from the little pier near the Aberdeen Marina Club or I think they are also at Aberdeen Promenade.