Facebook says getting children to communicate on its Messenger Kids app took nudging, even though experts they consulted for the product say kids are ready to use such mobile software.

When Facebook was testing Messenger Kids before rolling it out in December, the company found "It was hard for kids to initiate the communication," said Tarunya Govindarajan, the manager who built the product, to an audience gathered in Washington for the company's Global Safety Network Summit. "We wanted to give them nudges to start the conversation," she said.

So along with a visual chat feature and video content, Facebook built "kid-friendly effects" into the product, including augmented-reality photo filters like "stickers, masks and frames" to encourage kids to use it, Govindarajan said Thursday.

That suggests many children within that age group are not as ready to use mobile communications software as some have argued.

For example, Sean Arthurs, senior manager of education initiatives for the ‎National Parent Teacher Association, said at the same event that the group's research found 80 percent of children ages eight to 13 were using mobile communication devices.

"Ninety percent of kids have access to a phone or tablet and two-thirds have their own device," Arthurs, calling the results of the group's survey of 1200 parents "consistent with other research" about kids' usage habits.

The National PTA is among a group of organizations Facebook cited in support of Messenger Kids that also received funding from the company.

Facebook introduced the mobile application for iPhone and iPad users in December and an Android version is now available in the Google Play Store.

There's no advertising in the product and it does not require users to open a Facebook account. It allows parents to control who their kids communicate with using the app.

Yet a group of child-advocacy groups in January asked the company to discontinue it, citing multiple studies showing that heavy social media use had negative mental health effects on teens and adolescents.

When CNBC asked Facebook last month if it planned to continue the product, the company replied in an email that read in part:

"Since we launched in December we've heard from parents around the country that Messenger Kids has helped them stay in touch with their children and has enabled their children to video chat with fun masks. We worked to create Messenger Kids with an advisory committee of parenting and developmental experts, as well as with families themselves and in partnership with National PTA. We continue to be focused on making Messenger Kids be the best experience it can be for families."