Bangkok is always worth spending time in, not least for the food! The Grand Palace and Wat Po are definites, the kids would probably enjoy a long tail boat ride (the river is a great place to sit and watch the world go by). Sunday mornings in Lumpini Park are fun.
Avoid snake and crocodile farms, they are horrible. The musuem of Foresic Science is also horrible, but utterly fascinating for those with seriously strong stomachs.
At 7 and 10, definitely look at the Hyatt in Hua Hin and Chiang Mai elephant trekking / goat carts, rafting.
On a 14 day holiday, I would suggest
2-3 days in Bangkok, get used to the heat and over the jet lag
3-4 days Chiang Mai, getting out and about
5-6 days on the beach
Back to Bangkok for a shop and off
Flights - try Eva Air premium economy, business class seats, economy service. Also, flights stopping in the Gulf (Gult Air, Emirates etc) tend to be lower priced, good service but the stopover is a drag.