Hi Kells!
Great to hear back from you

Yes, William Wellman and Mary Orchard definitely seem like a good fit for Mary Wellman's parents, since she was of the parish Melbury Sampford. The date of their marriage aligns with the time I roughly estimated that Mary Wellman would have likely been born if, say, her marriage to David Childs happened when she was around 18 or 20.
Alternatively, I was looking tonight on the OPC Askerswell page and found that there was a high concentration of Wellmans there, including one 'Mary Wellman' christened 30 June 1746. Her parents' names were Robert and Margaret. I found that result by doing a search on the LDS Family Search. But, compared to the Wellman/Orchard theory this Askerswell theory is a longshot as I have no evidence to link Mary Wellman who married at Melbury Sampford with the Mary Wellman born at Askerswell...I wish there were records going back further. The Dorset OPC website is so amazing in terms of how accessible it makes the info that when you do finally get stuck it is very frustrating!
Another possibility is "Mary Welman" born to Elias Welman and Mary Welman, baptised 18 February 1740 at St Mary's Beaminster, Dorset. I've only just found that result, though, so I haven't looked into it any further other than to check the distance from Beaminster to Melbury Sampford on google maps. I got that result from the LDS search, too. When I went to check it on the Dorset OPC page for Beaminster, the transcriptions were wrong every time for that family; sometimes written as Wellin, Nellman and Wolman (with a ? typed after it).
Private message received! Thank you! I'll send you my email address too. Seems like we might have gone about as far as we can go with David Childs, though, unless some earlier records are made available.
Cheers,
Michaela