Making the following assumptions: 1 adult + 1 child (minimum group size for FAM Railcard to be valid), Return ticket required, and travel back also via Evesham, the fares are as follows:
Off Peak Day Rtn £35.30 (Rte Evesham) (unlikely to be useful for the 17:22/18:22 but shown for completeness)
Super Off Peak Rtn £40.80 (Rte Evesham/Stroud)
Off Peak Rtn £39.10 (Rte Evesham)
Anytime Rtn £61.20 (Rte Evesham)
Super Off Peak restrictions:
OUTWARD TRAVEL
Services via OXFORD:
Valid by any train.
(Restrictions apply if travelling via SWINDON)
As you are travelling via Oxford, and via Evesham, the Super Off Peak Return is the most appropriate fare for travel on either the 17:22 or 18:22. A much higher fare applies if travelling via Swindon. If the FGW site does did not show this fare for any trains via Oxford & Evesham then I would suggest a complaint be sent to FGW.