The CEO of online retailer Totsy.com has resigned. Guillaume Gauthereau, who co-founded the fast-growing private sale site for kids and moms, says he wants to pursue other interests and will remain on the board of directors. Lisa Kennedy, who has worked at Amazon.com Inc.-owned Quidsi Inc., will replace him as CEO.

“The company has grown very quickly since its founding in late 2009 and we are confident that Lisa will be able to continue and even accelerate that momentum,” Gauthereau says. “Like many entrepreneurs, my passion lies in building early-stage companies and I look forward to continuing to follow that passion through future endeavors while remaining on the board of Totsy.”

Kennedy joins Totsy from Hopscotch.in, an Indian flash sale site she founded last year that is similar to Totsy, whose products include apparel for children and moms, toys, games, books, and other child-related home products and supplies. Previously, from 2007 to 2011, she was executive vice president of e-commerce at Quidsi Inc., which runs a family of online stores including Diapers.com, Soap.com and BeautyBar.com. Amazon.com Inc. bought Quidsi in late 2010 for $500 million.  Also for a brief period in 2011, Kennedy served as CEO for VIP Art Fair, an online marketplace for art.

“I have a great deal of respect for Totsy’s accomplishments to date and am eager to work with the team to help aggressively fuel the company’s growth in the months and years to come,” Kennedy says.

Totsy is the second-fastest growing merchant in the Internet Retailer Second 500 Guide, which ranks and profiles the up-and-comers in e-commerce. The merchant increased sales about 549% from more than $2.11 million in 2010 to nearly $13.70 million in 2011. This year, Totsy will break into the updated Top 500 Guide, which is due out in late April.

Much of the retailer’s growth, Totsy has told Internet Retailer, has come from its social media marketing strategy, which seeks to link up with moms who spend time during the day on social networks, sharing images, stories and tips about motherhood. Totsy is No. 4 in the Social Media 300, which ranks online retailers by the percentage of traffic to their sites that comes from social networks.

In January, Totsy acquired competitor Mamasource.com, which brought the combined company to 5 million members. Terms of the deal were not disclosed. The merchant also entered the Chinese baby products market when it signed a deal early last year to sell its products through MamiBuys.com, a China-based flash sale e-retailer that imports baby products to sell to Chinese consumers.