Trying to duplicate the success of blockbuster Web sites like Club Penguin and Webkinz, children’s entertainment companies are greatly accelerating efforts to build virtual worlds for children. Media conglomerates in particular think these sites — part online role-playing game and part social scene — can deliver quick growth, help keep movie franchises alive and instill brand loyalty in a generation of new customers.

Second Life and other virtual worlds for grown-ups have enjoyed intense media attention in the last year but fallen far short of breathless expectations. The children’s versions are proving much more popular, to the dismay of some parents and child advocacy groups. Now the likes of the Walt Disney Company, which owns Club Penguin, are working at warp speed to pump out sister sites.

“Get ready for total inundation,” said Debra Aho Williamson, an analyst at the research firm eMarketer, who estimates that 20 million children will be members of a virtual world by 2011, up from 8.2 million today.

Worlds like Webkinz, where children care for stuffed animals that come to life, have become some of the Web’s fastest-growing businesses. More than six million unique visitors logged on to Webkinz in November, up 342 percent from November 2006, according to ComScore Media Metrix, a research firm.