It is unclear to me whether you simply have a Child::FieldName field on a parent layout, or whether you're referencing Child::FieldName in a calculation. If the former is true, you should actually see a list of all related FieldName records from the Child TO. If the later is true, yes, only the "first" child record is displayed.
1. If you want to collect all records in the child table from the parent table within a calculation, you can use List ( Child::FieldName ).
2. If you want to collect the Nth record in the child table from the parent table within a calculation, you can use GetNthRecord ( Child::FieldName ; N ).
3. If you want to change the way FileMaker determines which record is the "first" record in the relationship, you can look in the relationship details for a Sort option.