Apparently young kids are not getting enough of this.
It flags up a recent Unicef finding that British children are among the developing world's unhappiest.
The letter says: "We believe that a key factor in this disturbing trend is the marked decline over the last 15 years in children's play."
The experts say that play - especially when it takes place outdoors - is crucial to a child's health.
An increase in traffic, parental fears about abduction by strangers and a "test-driven" culture of education have all contributed to the trend, they insist.
They add that "the ready availability of sedentary, sometimes addictive screen-based entertainment and the aggressive marketing of over-elaborate, commercialised toys" have also played a part.
They call for a "wide-ranging and informed public dialogue about the intrinsic nature and value of play in children's healthy development".
There is so much pressure to watch TV, play on video games and computers and other electronic toys such as MP3 Players and Mobile Phones that do everything except feed and bathe you and taking care of poos and wees. There is no tactile experience there and we will be raising a generation of sedentary individuals, liable to be deaf and blind at 40.