Langkawi Island is nice but it isn't like Phuket so you may be disappointed if you're looking for stunning beaches, and things to do. On that note, Langkawi is very peaceful and quiet if that's what you're looking for.
I also like Penang which is slightly more 'lively' than Langkawi, but still not as great as Phuket in my opinion. Having said this, your kids will love
Penang, especially Georgetown and Batu Ferringhi where you can do plenty of water sports.