If this becomes law I suspect what will happen is that if someone books with a child under 13 and doesn’t pick a seat a middle seat is set to unavailable and their group goes on a list. Once the number of groups on the list matches the dwindling number of seats together they will assign the seats. After that, if a group attempts to buy a ticket for someone under 13 the plane will show as full, as there are no two seats together available anymore. Two adults will still be able to buy tickets for that flight, they just won’t be able to sit together. Families may find flights with a dozen aisle seat open, but they won’t be able to book.
And then the whole plane applauded.