Is this simply a panel with a 50 amp GFCI breaker in it?
You need a 4 wire feeder from the main panel connected to the 50 amp breaker. This will connect to the lugs in the Midwest panel. Be sure and keep the green ground and the white neutral separate in all panels after the main panel in the house but in only that panel the green and white connect together.
Now from the spa panel to the hot tub you need a 4 wire feeder, white and green can be #10 and the 2 power wires will be #6. Green connects to the ground in the hot tub and the spa panel, white connects to the white in the spa and to the center terminal marked white on the breaker in the spa panel. The 2 black wires connect to the outside terminals on the breaker and the black and red terminals in the hot tub. The trick is the white wire to the 3rd terminal on the breaker as the white breaker wire is already connected to the neutral buss in this spa panel