Get second level terms of custom taxonomy

I want to get second level terms of a specific parent (first-level) term in a custom taxonomy. Sounds complicated but would be useful.

Term 1
       SubTerm-1.1
       SubTerm-1.2
           SubTerm-1.2.1
    Term 2
       SubTerm-2.1

Say, if SubTerm->parent is Term 1’s id, then i want to output SubTerm 1.1 and 1.2 but not 1.2.1.

wp_list_categories(array('depth' => 2, 'taxonomy' => 'customtax')); is not i’m looking for as it lists terms with their links, which i dont want to, and there is no filtering option by SubTerm->parent id.

So how to achieve this?