Not everyone impacted by this two child UC cap is a lone parent. There are low earning families with both parents working, earning minimum wage, that qualify for UC.
Overall I'm on the fence with this. I don't think you should deliberately have more children than you can afford, but not all children are planned, contraception fails sometimes, and abortion isn't an option for everyone. What if you had 3 children because you had a good household income and one partner is made redundant or becomes unable to work, and then the family needs UC to supplement the single income they are left with? Do they pick their least favourite child to social services? It's not black & white.