You will be need to book the one points flight separately from the others. I would actually suggest you make 3 bookings:
-4 one way flights to the destination paid with cash
-3 one way flights returning, paid with cash
-1 one way flight returning, using RR points (this should be one of the adult's tickets)
Making 3 separate reservations is a bit more work, but doing this gives you the greatest flexibility to make changes and take advantage of any price drops. Prices are the same if you book one way or roundtrip.
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