A deceptive sign often seen in residential neighborhoods, intended to warn motorists to drive slowly because of the presence of children, but often humorously misconstrued as indicating the neighborhood youngsters' lack of agility. Though made to give the appearance of official traffic signs, "slow children," or sometimes "slow children at play" signs hold no legal weight.

Studies have shown that these signs don't decrease the average speed of traffic through neighborhoods where they're installed, so their efficacy as a safety feature is dubious. At the very least, signmakers should insert a comma to eliminate grammatical ambiguity. An alert driver will usually be able to maneuver safely around slow children. It's the fast ones that you can't see in time to avoid.

information from http://www.ci.phoenix.az.us/STREETS/atplay.html