I prefer Thailand to Malaysia - I don't know what it is exactly but Thailand seem to "do" tourism much better IMHO.
We live in Singapore at the moment so we've been to both Malaysia and Thailand a few times and have enjoyed Thailand more........however, we have not been to Langkawi and I have heard some good things about it.
With the West coast of most of that peninsular, April - October is the low season. I have holidayed in Phuket and Khao Lak during the low season and the sea is rough (too rough to swim for an adult, definitely not suitable for children). Often the hotels and resorts do some work during the low season so you can be lying by the pool to the sound of hammer drills. The weather can be great but if you're unlucky you could have a rainy week - there was some flooding last year during low season.
The last place we went was the Ramada in Khao Lak - it was a lovely hotel, a bit of a quiet area though - not much to do unless you hire a car or are content to hang around the hotel and small town/village (it had a kids club and some activities but some were an extra charge). It was just at the end of October and we did notice a considerable difference in the calmness of the sea towards the end of our stay - it was quite amazing really how much it does change.
If you don't mind a quiet place, then Khao Lak is worth a look. If you prefer something a bit busier then Phuket still has some nice places although I would avoid Patong - even without my children in tow I find it seedy. The south of Phuket is nicer - Nai Harn beach and Kata in particular are my favourites. Kata Thani usually gets good reviews but we often stay somewhere smaller in Phuket (places without kids clubs)
Ao nang and Railey are picturesque but I'm not sure how you'd feel about travelling in a long-tail boat with a baby and small children - the only way to get to Railay is via boat. But you could stay at Ao Nang which you can reach by car - it doesn't have a great beach but it has a nice feel to it with a promenade style front and it's easier to get about on the mainland if you stay in Ao Nang (hotels are cheaper too).
Koh Lanta is lovely too - quite underdeveloped when I last went but that was about 6 years ago and it is much more popular now so I'm not sure what it would be like.
The only inclusive place I can think of is club med who have a resort at Kata Beach on Phuket. The thing I feel about Club Med though is that if it's in the middle of nowhere (such as the one on Bintan in Indonesia) then it's great because there isn't anywhere else to eat, but with a place like Kata on Phuket, it's surrounded by good restaurants and things so it seems a bit of a waste to stay somewhere like that and never go anywhere else.
You could consider Bintan actually if you want all-inclusive - fly to Singapore and get the ferry over (these are quite nice ferries - not the dodgy rickety things you hear about sinking in Indonesia!). Avoid Singapore school holidays though as it gets packed (you can check them out on the MOE - Ministry of Education - website for Singapore)
Most hotels I stay at I review on tripadvisor - I'm called BigSisSingapore on there, so if you want to check out my reviews please do :)