FTC chairman Andrew Ferguson said the inquiry will "help us better understand how AI firms are developing their products and the steps they are taking to protect children."
But he added the regulator would ensure that "the United States maintains its role as a global leader in this new and exciting industry."
Character.ai told Reuters it welcomed the chance to share insight with regulators, while Snap said it supported "thoughtful development" of AI that balances innovation with safety.
OpenAI has acknowledged weaknesses in its protections, noting they are less reliable in long conversations.
The move follows lawsuits against AI companies by families who say their teenage children died by suicide after prolonged conversations with chatbots.
In California, the parents of 16-year-old Adam Raine are suing OpenAI over his death, alleging its chatbot, ChatGPT, encouraged him to take his own life.
They argue ChatGPT validated his "most harmful and self-destructive thoughts".
OpenAI said in August that it was reviewing the filing.
"We extend our deepest sympathies to the Raine family during this difficult time," the company said.
Meta has also faced criticism after it was revealed internal guidelines once permitted AI companions to have "romantic or sensual" conversations with minors.
The FTC's orders request information from the companies about their practices including how they develop and approve characters, measure their impacts on children and enforce age restrictions.
Its authority allows broad fact-finding without launching enforcement action.
The regulator says it also wants to understand how firms balance profit-making with safeguards, how parents are informed and whether vulnerable users are adequately protected.
