Not sure 24 can be classed as a 'kid' for rail fares.

Assuming that's not a typo....
At the moment, your cheapest option is probably Advance Purchase (AP).
Looking at your dates and times and using
FGWs▸ booking site (online discount for AP) cheapest I've found:
1st November0802 Thatcham - Cardiff Central 1023 (change Reading) 3 adults 1 child
AP ^78.80 (3x ^22.50 1x ^11.30)
1825 Cardiff Central - Thatcham 2103 (change Reading 3 adults 1 child
AP ^47.30 (3x 13.50 1x ^6.80)
Total ^126.10If you had a Family & Friends Railcard that would come down to a total of ^78.30, so the ^30* outlay for the Railcard would still show a saving on this one journey alone. If you do decide to go for the discounted APs there's nothing stopping you buying them online now whilst waiting for the F&F Railcard (should you purchase) to arrive. Plenty of time between now and travel date.
All prices correct as at 1300 21/09/2013. As I've quoted AP prices, they are liable to change.
*You can buy a F&F Railcard online for ^27 at the moment using promo codes LEAFLET3 or FAMILY10 (see
here)
* If you have ^15 of Tesco Clubcard Vouchers you can get a F&F Railcard for 'free' (see
here)
Whatever you decide, and if it really is 2 adults 2 children (which means I'll have to re-quote

), don't use the thetrainline.com for FGW Advance Purchase fares. Buy direct from firstgreatwestern.co.uk. First, you'll get FGWs online discount for APs, and second, FGW won't charge you booking or card fees.