Big-box retailers such as Target and Walmart are capitalizing on the demise of Toys R Us this holiday season, and it's paying off, according to industry leaders.

The well-known toy retailer, which shuttered its U.S. stores in June, accounted for 15 to 20 percent of the country's total toy sales in 2017, based on estimates from Jefferies. Some retail experts worried that its liquidation would leave a sizable gap in toy sales this holiday season because less-specialized retailers weren't as committed to the category.

But early data and anecdotal evidence suggest that's not how early holiday sales are playing out at national chains.

Bill Simon, the former CEO of Walmart, told CNBC's "Squawk on the Street" that it was "exciting" to watch the "battle for the toy business" as Thanksgiving Day sales and early Black Friday promotions got underway Thursday evening.

"I thought Target was a winner last night in the toy area," he said on Friday. "I think the demand for toys continues. I think that there'll be a lot of people that get engaged and in fact saw toys in a lot of the retail Black Friday events in places that you wouldn't have guessed."

Department stores, grocery chains and drugstores have reportedly allocated room for toys on their shelves in the wake of the bankruptcy. Best Buy, an electronics retailer, has begun selling Mattel's Barbie dolls.

Even athletic stores such as Academy Sports + Outdoors, a discount sporting goods chain owned by KKR & Co., Simon's current employer, stocked toys on their shelves, the former Walmart chief said.

Still, experts agreed that big-box operators such as Target, Walmart and Costco, not to mention e-commerce giant Amazon, are the biggest winners of the Toys R Us fallout.

It will undoubtedly "help the Targets and Walmarts of the world," Women's Wear Daily editor James Fallon said on "Squawk Box," adding that the recent reopening of famed toy seller FAO Schwarz underscored the still-powerful demand for toys.

Allen Questrom, former CEO of J.C. Penney, told "Squawk Box" that store closures around the country have been a "big, big plus" for larger retailers, not just in the shifting toy space but also in apparel, where closings of smaller department stores such as Bon-Ton are giving operators such as Macy's — which has also been reducing its store count — a boost.

But while store closures have compressed the square footage at retail's upper ranks, they haven't necessarily weighed on major public operators' profitability, Ron Johnson, the founder and CEO of Enjoy, said Friday.

"This is the first time in nearly two decades that the physical stores have the upper hand as we head into the holidays," Johnson told "Squawk on the Street." "You see that at Target. ... They've turned the stores from what people thought was an anchor into the engine of their omni-channel strategy."

Retail, the fellow J.C. Penney veteran added, is "a really good business for the survivors."

Even so, the CEO of MGA Entertainment — the company behind L.O.L. Surprise, one of the year's hottest toys — harbored concerns that no single retailer, or even a combination, could come close to fully replacing Toys R Us.

"I'm afraid that nobody's going to fill the whole void, and the only reason for that is Toys R Us only sold toys," Isaac Larian explained on "Closing Bell." "In December, they had all the hot toys, but when you go to January, most retailers are going to cut back on their inventory, so there's going to be [a] shortage of merchandise, especially hot toys, come this Christmas."

Shares of Walmart rose nearly 1.5 percent in intraday trading on Friday as consumers took advantage of online and in-store Black Friday sales. Target's stock, still under pressure from the company's muted third-quarter earnings report, shed more than 2 percent intraday. Costco shares rose 1 percent.

October court filings revealed that Toys R Us' top lenders have cancelled the bankruptcy auction in the hopes of reviving the brand and maintaining the brand's global license agreements. Toys R Us filed for Chapter 11 bankruptcy protection in September 2017.