Don't know if the OP is still reading, however I would suggest the buffet at the top of baiyoke tower, formerly the tallest building in bkk. The city views are good and the buffet is cheap with solid "B" food and a fairly wide spread of cuisines. Depending on the age of the children, there is an option to head outside and take in the city views from an open air rotating observation deck.

EDIT: would also suggest the restaurant on the 25th floor of the Grand China Hotel in Chinatown. Has a nice lounge-y vibe and is convenient if you've been sightseeing.

Have a great trip!