Through fare tickets from Tiverton Parkway - Bristol Temple Meads at the times you want will cost you ^44.00. That's ^22.00 for an adult and ^11.00 each per child.
The first service you can catch with those fares (Off Peak Returns -SVRs) is the 0837 CrossCountry service, arriving Bristol at 0927. Returning around 1600 you have a
FGW▸ service from Bristol
TM‡ at 1553 which requires a change at Taunton (5 minute wait) to another FGW service arriving Tiverton Parkway at 1718. Or there are direct CrossCountry services at 1544 or 1645 arriving 1628 and 1729 respectively.
Slightly cheaper fares can be had by splitting your tickets at Taunton.
Tiverton Parkway - Taunton
Anytime Day Return 1Ad 2 Ch. ^18.60*
Taunton - Bristol Temple Meads
Off Peak Day Return 1Ad 2Ch ^22.00
*Off Peak Day Return is ^17.20 but only valid after 0930.Total ^40.60.
If you require bus travel around Bristol for the day you can also buy PlusBus tickets for ^3.50 Adult and ^1.75 Child. Giving unlimited travel in the Bristol area on most operators. PlusBus is available from the ticket office when you purchase your train tickets.
http://www.plusbus.info/bristol-plusbusWhilst you won't make a saving with this journey alone if you had a Family & Friends Railcard, it may be worth looking into if you plan on making further trips with at least 1 adult and 1 child together. The Railcard costs ^28 and gives you 34% off most standard class adult fares as well as 60% off the child fares.
http://www.familyandfriends-railcard.co.uk/For reference, the through fare I quoted above at ^44.00, would cost ^22.90 with the Railcard, saving ^22.10. The split fares would cost you ^20.45 saving ^20.15. As you can see this one trip would almost pay for the Railcard. Another similar trip and you start making real savings. F&F Railcard also gives you the same discounts on PlusBus tickets - although the child fares will usually cap at ^1 each as that is the minimum fare with the Railcard. Thus bus travel in Bristol will cost ^4.30 for the three of you, saving another ^2.70.
