I've live in Malaysia for 4 years, but only visited Langkawi this year. I thought it might be a bit too pedestrian and boring. BUT I was massively wrong. We've been three times this year (its not very far on a flight from KL). and it is simply brilliant with kids, probably of all ages. We've tried it with toddlers and with teenagers.
The three upmarket places to stay are
Datai, Andaman and Tanjong Rhu. All very nice. Good rooms. good service, lovely beaches etc etc. BUT I always feel a little nervous about the kids making too much noise, so it is hard to relax totally.
After that, we have stayed at the Pelangi Beach, which was surprisingly nice. Its downmarket and significantly cheaper than the three above, but the rooms are nice and in small units, either facing the sea or little lakes and there is a great children's club. The food was a bit patchy, but hey there was a swim up bar and you can walk down the beach to the other places like Casa Del Mar and a few others that do better food.
My current favorite is a place that started as a restaurant, Bon Ton. The food is brilliant and it has been very stylishly turned into a small hotel with seven traditional village houses (the ones on stilts). Utterly relaxing. None of the sometimes obsequious but faceless service of the bigger places. Also none of the crowds. BUt it does lack a direct beach. THe best point about the place is booking all the houses and having a group holiday in what is effectively your private resort...
As for things to do in Langkawi, there is a German chap called Juergen who takes you abseiling through the jungle. Quite terrifying and exhilarating at the same time. ANd there are some great yachts to charter for the day.
SO, you see I am quite a convert. BUT I wouldn't stay in a Berjaya resort if I could help it. I haven't tried out the Langkawi one, but the others are of the "keep-it-cheap, pack-em-in" variety.