Important- From 1 February 2019, the government is-
1. removing the UC gateway that prevented claims for Universal Credit where a client has three or more children. And also-
2. not imposing planned restrictions on some claimants where their 3+ children were born pre 6/4/2017. This will allow the payment of child element in new claims to Universal Credit for all children born before 6 April 2017 (where entitlement exists).
Here's the law and the guidance.