One of the weaknesses of traditional workflows within Dynamics 365 is their inability to natively iterate over child records. Great news! Microsoft Flow allows you to be able to iterate over child records and pass in query string parameters allowing you to limit the returned collection. It achieves this using OData syntax. Trust me though, even if you have never used OData syntax before, with a bit of trial and error you’ll pick it up in no time. Check out this post for a very quick start to iterating over child entities using Microsoft Flow.