How to display all children on parent node for custom content type?
How are the children linked to the parent? Do you use an Entity Reference field?

Also, what do you want to display from the children? The titles? The entire, rendered nodes?

If you look up entity_metadata_wrapper (see: Entity metadata wrappers) you can see how to access an Entity Reference field.

You also may want to reconsider Views. Overriding templates for Views can give you a lot of flexibility.