You need to make a join. In the criteria API, the easiest way to do that is to use the createAlias() method:
session.createCriteria(Child.class, "child")
.createAlias("child.parent", "theParent")
.add(Restrictions.eq("theParent.someField", theFieldValue))
.list();
This query is equivalent to the following HQL:
select child from Child child
inner join child.parent theParent
where theParent.someField = :theFieldValue