It is clean and relatily safe. I would not be taking the kids down during the evening evern though there is nothing on display. Just be a lot of explaining.
Nana Plaza (officially Nana Entertainment Plaza; shortened NEP) is a 3 storey red-light district in Bangkok, which is rumoured to be the largest entertainment complex in the world. It is located on Sukhumvit Road Soi 4 across from the Nana Hotel, within walking distance of the BTS Skytrain's "Nana Station". The name originates from the Nana family (with Lek Nana the most prominent member) which owns much property in the area and has still much influence in Thailand.
The Plaza is in the shape of a square, with a single opening on the western side, and consists of a ground floor and two additional floors. It started as a restaurant area in the late 1970s. During the early 1980s, a few gogo bars appeared and gradually replaced the restaurants. The composition of the bars has changed over the past few years; the last non-go-go bar, the Woodstock Pub was sold in 2005 to the Rainbow Group and reopened as "Rainbow 4." A few bars offer a more pub-like or beer bar format without dancing; as of late-2009 only the Cathouse remained.