This video is produced by the Minnesota Local Road Research Board, and explains which warning signs are necessary and effective, and which may be counterproductive.
Children at Play signs
Some cities have posted “CHILDREN AT PLAY” signs in residential areas despite studies showing that generalized signs warning of Normal Conditions such as children in a residential area fail to achieve the desired safety benefits. "CHILDREN AT PLAY" signs may give parents and children a false sense of security as the sign is assumed to provide protection which in reality it does not. Due to these serious considerations, federal standards no longer recommend the use of "CHILDREN AT PLAY" signs. Special conditions such as warnings of school zones, playgrounds, parks and other recreational facilities, do warrant signing as these are not easily anticipated by drivers.