Fences and small incidental structures used for children's recreation shall not be subject to the requirements for the principal building or the accessory building, but no "spite fence" shall be erected or maintained. For the purposes of this Zoning Ordinance, a "spite fence" is any fence of such height or character as to block access of light and air to any ground floor window or upper window or to interfere with the reasonable use of a person's property. In any case, no fence shall be erected higher than five feet.