A few suggestions, some which we have done with 3 kids (they're now 7, 4 and 4 and when we got here they were 4, 1 and 1):
- Kota Kinabalu (one of the Shangri La's, we've done Tangjung Aru at Xmas 2 years ago)
- Cebu (also Shangri La's, been 2 1/2 years ago, off season)
- Hua Hin, 2 hour ride from Bangkok so many flights, fairly affordable and good range of hotels
- Vietnam, we've done Hoi An (Victoria), which is a 1-hour flight from either Hanoi / HCMC (very nice, good mix of culture and beach, not too crowded)
- October break, we're going to Panglao Island (Philippines), but have to change in Manila (can also take ferry from Cebu, but I am too scared for that).
- Vietnam, Phan Tiet / Mui Ne as somebody recommended, drive from HCMC.
- Bali, would probably be pricier
- Hainan, but I don't hear great things, good hotels though, and closeby. May not be really warm in December, not sure.
- Malaysia, Penang or Langkawi, I've done both prior to having kids, Langkawi even 21 years ago (with one 5* hotel). Penang has direct flights, Langkawi is a change in KL.
For some reason, Phuket doesn't sound that appealing to me (my parents didn't think it was great either)