The only cheaper way to do it is to travel out on the very first train (0719 ex Newbury, arriving Exeter St David's 1015, with a change at Reading), which would cost £19 for each of the adults.
Then return on any of the following services for £32.50, making a total of £51.50 per adult. (The 4yo child travels free anyway):
1644 (arr 1833, direct)
1852 (arr 2028, direct)
1917 (arr 2205, change Reading)
2031 (arr 2219, direct)
That, of course, assumes that the adults aren't eligible for Senior Railcards (requiring them to be at least 60 years of age).
Alternatively, if it's likely that the adults with be travelling with the child (or, indeed, with up to 4 children and with up to 4 adults in the party) in the future, then it might be worth investigating getting a Family & Friends Railcard. (Note that, whereas TWO Senior Railcards would be required for both adults to get discounted fares, only ONE Family & Friends Railcard would be needed). To make the card valid, a ticket would need to be purchased for the child but the adults would only pay 66% of the normal fares and the child would only pay 40% of the child fare (= 20% of the adult fare). A Family & Friends Railcard costs £30 per year.