Second what's been said about the Datai and the Andaman. we love it. Been 3 times. They have a lot of return guests.
The Datai is in a class of it's own and if your children can entertain themselves would recommend staying there to experience the full wow factor of the constant sights and sounds of the jungle.
You are allowed to use the facilities of the Andaman (sister property). That is a very child friendly swimming pool with aqua slides and a kids club.
Both have dirt cheap kids menus and babysitting (considering the standard).
For amateurs, the Datai has one of the best priced french wine list I've seen and food is quite reasonably priced.
Activities include mini hikes to lovely fresh water waterfalls, evening and early morning nature trails with resident biologist and for the fitter a day mountain treck.
There is a marine park 1 hour boat drive to the south of the island. Gorgeous day out. Fab diving.
They also have one of the most beautifull golf courses in the world.
The recently opened Casa Del Mar has excellent reviews in Asian travel mags. The Four seasons is "tacky" compared to the Datai say afficionados. The Tanjung Rhu still scores well. Can't give you first hand advice as we arre Datai die hards and don't bother with the rest.
In Langkawi, tourism is definitely either high end or very tired old 2/3 stars with nearly nothing in between.
I think it's very easy travel between langkawi and Penang (if you don't mind the packing and unpacking).
Don't know Penang that well but definitely more choice for hotels and maybe more opportunities to entertain the children with outings and a bit of culture, although not much really.
If ou are looking for an all inclusive with lots of children's entertainment and evenings out, then Penang may be a better option.
Have you considered Krabi/Khao lak/koh Lanta? It's beautifull and very easy to fly to. And best for boating (day trips to phi phi island and marine conservation park)
For accomodation try asiarooms.com - they have the best local deals
For flights try tiger airways and silk air from singapore and bangkok air from thailand. - they are the local easy jets.