A group of Democratic lawmakers wrote to Mark Zuckerberg this week to press the CEO on his plans to curate a version of Instagram for children. In a hearing last month, Zuckerberg confirmed reporting by BuzzFeed that the company was exploring an age-gated version of its app designed for young users.

Senators Ed Markey (D-MA), Richard Blumenthal (D-CT) and Representatives Lori Trahan (D-MA) and Kathy Castor (D-FL) signed the letter, expressing “serious concerns” about the company’s ability to protect the privacy and well-being of young users.

“Facebook has an obligation to ensure that any new platforms or projects targeting children put those users’ welfare first, and we are skeptical that Facebook is prepared to fulfill this obligation,” the lawmakers wrote.

They cited previous failures with products like Messenger Kids, which had a flaw that allowed kids to chat with people beyond their privacy parameters.

“Although software bugs are common, this episode illustrated the privacy threats to children online and evidenced Facebook’s inability to protect the kids the company actively invited onto this platform,” the lawmakers wrote.

“In light of these and other previous privacy and security issues on Facebook’s platforms, we are not confident that Facebook will be able to adequately protect children’s privacy on a version of Instagram for young users.”

The letter set a deadline of April 26 for the company to provide answers to a comprehensive and helpfully specific set of questions about a future kid-targeted product.

In the letter, lawmakers posed a number of questions about how Facebook will handle the private data for young users and if that data would be deleted when an account is terminated. They also asked the company to commit to not targeting kids with ads and not employing push alerts and behavior-shaping features designed to make apps more addictive.

During last month’s big tech hearing in the House, committee members from both political parties grilled Zuckerberg about how Facebook and Instagram adversely affect mental health in young users. Rep. Castor also pressed the chief executive about underage users who circumvent Instagram’s existing age guidelines to use a platform full of posts, videos and ads designed for adults.

“Of course, every parent knows there are kids under the age of 13 on Instagram, and the problem is that you know it,” Zuckerberg said.