How to use CiviCRM for child Sponsorship?
I like Graham's solution of using Personal Campaign Pages with one page per child.  However, that may not scale well; I don't know how many orphans you're working with.

I like Graham's idea of using Webform-CiviCRM on Drupal (or Caldera Forms integration on WordPress) to handle this.  I think creating a Relationship would be the easiest approach if you go this route rather than a soft credit.  I'm unsure whether you want to create the connection between the donor and orphan, or the donation and the orphan.

If neither of these options appeal to you, you can use a Contact Reference custom field to help create the relationship.  However, if you want this to happen automatically, you would need to write an extension.  I recommend using the hook_civicrm_post hook.