Hello Kelly,
I am the five times great granddaughter of Lydia Childs, the firstborn daughter of David Childs of Melbury Osmond who was transported to Australia on the Earl Cornwallis as a convict. I, too, have had difficulty working out David's parentage. On my current ancestry.com tree, I've got Joseph married in Axmouth Devon as his father but, like you, couldn't ignore the fact that David is not mentioned in the 1770 will when the other children are.
As the postings on this thread are quite old, I wondered if you ended up uncovering any further information about David's parentage?
I noticed a lot of theorising about whether Samuel and David were connected in any way...It's interesting that Lydia called her second Australian-born son "Samuel" because her first born son was clearly named after her father as he was given the name "David Childs Barber."
I also wasn't able to trace David's wife Mary Wellman even though their marriage record states her parish was Melbury Sampford...Perhaps it was a case of her not actually being of the parish since birth but only for the minimum of three weeks prior to the marriage...
Look forward to hearing from you.
Cheers,
Michaela