You can still use get_posts
:
$parents = get_posts(
array(
'posts_per_page' => -1,
'post_type' => 'my_custom_post_type',
'fields' => 'ids', // Just get IDs, not objects
'tax_query' => array(
array(
'taxonomy' => 'my_custom_tax',
'field' => 'slug',
'terms' => array(
'foo',
'bar',
),
),
),
)
);
$post_ids = get_posts(
array(
'posts_per_page' => -1,
'post_parent__in' => $parents,
'post_type' => 'my_custom_post_type',
'fields' => 'ids',
'meta_query' => array(
array(
'key' => 'my_custom_field',
'value' => $foobar,
),
),
)
);
Further reading on tax queries and meta queries.