Formula field looking up Child of input

Formula field looking up Child of input

I have two forms.
  1. Clients - Has First_Name and Last_Name fields.
  2. Attendance - Has Client lookup field, Date, and Status.
I want to add a formula field to Attendance that will concatenate Client.First_Name + " - " + Status
When I try this, the Client.First_Name I get 'Client' is a lookup field and child fields cannot be accessed
When I try Clients.First_Name I get   Variable 'Clients' is not defined
When I try input.Client.First_Name, it accepts it, but returns null for input.Client.First_Name
If I only put in Client it returns the ClientsID.