Take the whole family to CW for the visa. There are always lots of restaurants there, a big indoor open area where the kids could play. Sometimes there are expos of stuff from provinces and everybody can poke around and see some of the differences between Thai regions.
On a 60-day tourist visa, you can extend in the last 45 days -- after 15 days from arrival) -- Only at Chaeng wattana. I did it and had 44 days without a thought about a visa.
Bus 66 goes there. It goes from Southern Bus Terminal to The Government Complex via Purple MRT Tao Poon (Blue MRT > Bang Sue > shuttle to Tao Poon station > walk 5 minutes to bus stop > Bus 66 to immigration)