But this is already the case - as long as your children were born before 06/04/2017!!
If someone lost their income and had to claim benefits tomorrow, they get child element for any children before that date. If they have multiple children born after that date when not on benefits and then need to claim UC, they will be subject to the 2 child cap unless they meet the conditions to qualify for an exemption - children born as part of a multiple birth pregnancy, children who are under a guardianship order, if they are placed with you by social service, if they are the child of one of your children under 16, if you have adopted them, if they are born as a result of rape (where you do not live with the abuser) - or a few other specific situations.
But, if people have had more than 2 children since 06/04/0217, and later need to claim benefits, they have to accept they will not receive
money for all of them.