There's nothing in the legislation that requires DOT actually implement a policy. Note the "if appropriate" part below. Also, it doesn't require the entire family to be seated together. It still permits a family to be split up as long as children 13 or under are seated next to a family member who is over 13. With two adults, it would be permissible to put two kids next to one adult and one kid next to the other.
Yes, hence the ‘splaining if and when something bad happens. “Secretary Chao, why did you deem it not appropriate to implement a policy to ensure that a 3-year-old wouldn’t sit by herself next to a ....”?
Of course, one should also question why they passed such a vague law in the first place.