Formula field with Child dependency

Formula field with Child dependency

I have two forms, Clients and Attendance.

Clients has First_Name and Last_Name fields.
Attendance has Client, Date, and Status fields.  The Client field is a lookup of Client.First_Name + Client.Last_Name 

I want to add a formula field to my Attendance form to combine Client.First_Name + " - " + Status

If my formula is;
Client.First_Name + " - " + input.Status   'Client' is a lookup field and child fields cannot be accessed
Clients.First_Name + " - " + input.Status   Variable 'Clients' is not defined
input.Client.First_Name + " - " + input.Status returns "null - Absent" 
Client returns the ClientID