Family Evenings in Bangkok
There are a reasonable but not overwhelming number of things that families can do in the evening in Bangkok. Most restaurants are very family friendly and it is not unusual in more informal establishments for a waiter or waitress to keep a child entertained a bit. Most restaurants don’t mind if the child roams around a little either.

There are also a number of ways to enjoy a meal in Bangkok while taking a cruise on the Chao Phraya River. Prices can range anywhere from 100 baht per person by hiring a boat under the Phra Ram IX Bridge to over a thousand baht person for a three-hour dinner cruise from the Marriott Royal Garden Riverside Hotel. A travel agent or your own hotel can probably help with arrangements too.

Markets
The Suan Lum Night Market, opposite Lumpini Park, has a vast array of goods to check out.

English Language movies
English language movies are available at several Bangkok locations including SF Cinema City in MBK Centre, United Artists Theatre in The Emporium on Sukhumvit Soi 24 and Lido at Siam Square.

Dreamworld
On weekends, Dreamworld, a kind of Thai "Disneyland" is open until 7 PM. In addition, many of Bangkok's large shopping malls have an area with kids rides and games.

Skytrain
A family nighttime Skytrain ride might be enjoyable. Hop on, ride to the end of the line and see the lights of Bangkok. See it from the other direction when you go back or perhaps take a tuk-tuk.