It is usual for cohabitation agreements to include something in them which tells both parties that they need to review the agreement when children become involved.
If you would like to discuss your cohabitation agreement after you have had children then you should both get independent legal advice. It is possible for you to add something into your cohabitation agreement that would mention the level of child maintenance that would be needed for the main carer if you and your partner separated.
Courts are enforcing cohabitation agreements more and more and it is a good idea that you modify it when you have children. However, if you do separate, the court may choose not to enforce the child maintenance level in your cohabitation agreement. The paramount consideration of the court when couples with children separate is the welfare of the children and they will not enforce any agreement that seems to go against this.