To eager fetch an association, you put a JOIN FETCH in the From clause. It can be LEFT or INNER join fetches.
so HQL would look like
Select p FROM Parent p LEFT JOIN FETCH p.children
One caveat is that you can only eager fetch one OneToMany or ManyToMany association. This is because if you do more than one you get a cartesian product.
You can do one Left and one Inner join fetch of OneToMany or ManyToMany association, but be sure that for the Many side, there is a guarantee that there is always at least one returned or the Parent doesn't load in the query. Just like an inner join in SQL resultset would do.
You can also look into the Fetching strategy of "subselect" so that the Many side association is loaded in just one more query rather than N number of queries.