Maternity leave, paternity leave and adoption leave are all examples of child related leave.
As long as you are being paid during any periods of child related leave (both ordinary and additional), you don’t lose any pension benefits. Even if this is a reduced pay, you carry on building up the same benefits you were doing before - in other words either standard benefits or 50/50 benefits, depending on which option you were in.
If you then go on to take some unpaid additional child related leave, you won’t build up any pension benefits. But if you like, you can catch up on the lost benefits by paying additional pension contributions (APCs) and your employer will share the cost with you by paying 2/3 of the cost.
But you only have 30 days from returning to work to take up this option. If you choose to pay APCs outside this deadline, you will have to pay the full cost.