To seek sole care of the children you would need to demonstrate an existing risk presented by the other parent and that as a result of that risk it is not in the children's best interests to see or spend time with the other parent at all.
This is not a decision easily taken by the Court and if a risk can be mitigated with by example supervision, drug / alcohol tests, parenting courses and the like the Court may still determine that the children should spend time with both parents.
You will need to gather evidence of the risks which may include issuing Subpoenas, obtaining reports from appropriate medical practitioners and outline in detail in your Affidavit what the risk is so the Court can make a determination.
If you have any questions about this information or would like to talk to a Solicitor about seeking sole care over your child(ren), please contact us to schedule your initial consultation.