You may alreadyhave this, Sam's household including his widowed Grandfather Webb, from 1880 census Dallas Co, TX:
Categories: Name; Relationship to head of household; Marital Status; Gender; Race; Age; Birthplace; Occupation; Father's Birthplace; Mother's Birthplace.
Samul A. COATS1SelfMMaleW20TXFarmerTNTX Mary COATSWifeMFemaleW18TXKeeping HouseTNTN Loyd COATSSonSMaleW1TXTXTX James PAYNEOtherSMaleW19ARWorks On FarmALAL Alexander WEBBGFatherWMaleW70OHNo OccupationUNKNOWNUNKNOWN
Looks like there is another child missing from you list, named Loyd.Or, he died young, and doesn't show up later on.
I would look for Alexander Webb on earlier census records.