The sight of a beaming Brooke Shields frolicking in playgrounds with her daughters, Rowan, 3, and Grier, 9 months, has become a familiar one in L.A. But these tots' easygoing public lives are a stark contrast to that of Suri Cruise, the daughter of Brooke's new friend, Katie Holmes!

"That child is growing up inside a bubble," says a source. "I think both dad [Tom Cruise] and Katie are so worried that if Suri is exposed to the public, her life could be in danger." Indeed. Suri was born on the same day (April 18) and in the same L.A. hospital as little Grier. Yet the child has hardly ever been photographed out in Beverly Hills — or, for that matter, anywhere else in the United States!

After appearing with her parents on the cover of the October issue of Vanity Fair, Suri did make "coming out" appearances in late November at Katie and Tom's gala Italian nuptials — but there, the family, which also includes Tom's adopted children, Connor, 11, and Isabella, 14, was protected by at least 200 police officers, plus a special detail of the crack Italian security force, the Caribinieri!

The last time she was seen was in her father's arms surrounded by eight bodyguards at an airport in the Maldives Islands on her parents' honeymoon. But in the two months since that getaway? Not one public sighting.

TOMKAT'S NIGHTMARE
Why so much secrecy surrounding a tiny child? Sources believe that Katie, 28, and Tom, 44, fear that all their children could be prime targets for kidnapping plots! "The world was so obsessed with getting its first peek at Suri, I think Tom and Katie might have decided to keep her under wraps to prevent some wacko from trying to abduct her," says a source.

No wonder. In March 2005, cops foiled a plan to nab David Letterman's then- 16-month-old son, Harry, for a $5 million ransom. Katie's new BFF, London-based mom-of-three Victoria Beckham, was herself the object of foiled kidnapping plots in both 1999 and 2002. And in 2001, the FBI investigated a kidnapping plot supposedly launched by Osama bin Laden's al-Qaida network focused on Russell Crowe.