@NWonders it seems you’re missing an {% endif %} on the child_row condition
Also, you might want to try selectattr here. I think foreign ID columns might only be queried by ID? You’ll have to experiment with it a bit for that.
Something like this might work:
{% for child_row in child_rows|selectattr('foreign_id', 'eq', row.hs_id) %}
<h6>
{{ child_row.company_name }}
</h6>
{% endfor %}
Best way to debug an error like this is to just constantly dump data onto the page to see what you get.