During ordinary parental leave, you'll pay contributions on the actual pay you receive.
However, for unpaid period of childcare leave, you can buy back this period. It will often be a small amount of pension “lost” in respect of this unpaid period. This is done through an Additional Pension Contract (APC).
For more information regarding buying back periods of lost pension, visit the APC modeller at LGPS 2015 APC Modellers.