var query = session.QueryOver<ParentEntity>(() => pAlias).Where(() => pAlias.Id == key).Future<ParentEntity>();

var queryA = session.QueryOver<ParentEntity>(() => pAlias).Left.JoinAlias(x => x.ChildEntitysA, () => caAlias).Where(() => pAlias.Id == key).Future<ParentEntity>();

var queryB = session.QueryOver<ParentEntity>(() => pAlias).Left.JoinAlias(x => x.ChildEntitysB, () => cbAlias).Where(() => pAlias.Id == key).Future<ParentEntity>();

var queryC = session.QueryOver<ParentEntity>(() => pAlias).Left.JoinAlias(x => x.ChildEntitysC, () => ccAlias).Where(() => pAlias.Id == key).Future<ParentEntity>();

return query.ToList();

SELECT parent.col0, parent.col1, parent.col2, parent.col3, .... parent.col99
FROM parent WHERE .....

SELECT  parent.col0, parent.col1, parent.col2, parent.col3, .... parent.col99, childA.col0, childA.col1, childA.col2 .... childA.col99
FROM parent LEFT OUTER JOIN childA ON ....

SELECT  parent.col0, parent.col1, parent.col2, parent.col3, .... parent.col99, childB.col0, childB.col1, childB.col2 .... childB.col99
FROM parent LEFT OUTER JOIN childB ON ....

SELECT  parent.col0, parent.col1, parent.col2, parent.col3, .... parent.col99, childC.col0, childC.col1, childC.col2 .... childC.col99
FROM parent LEFT OUTER JOIN childC ON ....