It did reduce it, by the equivalent of 5600 kids per year according to the Child poverty action group. So over the last 7 years since it came in that would be nearly forty thousand kids.
Unfortunately still plenty of people having 'shock' pregnancies, not knowing the cap existed until after they had their third, having children without worrying about whether they could afford them or not, or changing circumstances that they weren't prepared for (according to the same website when they did a survey).
No one should be having three or more kids in this day and age IMO, the planet doesn't need it, you don't know when your circumstances might change and nobody 'needs' 3 or more kids.
Giving more money to these families definitely isn't the answer, more youth clubs, more mental health provision, more early years education, more free school meals during school and the holidays, more mentors, more family hubs. That would be much more beneficial to these kids than handing their parents money and hoping they start making good decisions with it.