Yeah, the extra bit of detail theorized there is that atta came from children's vocabulary and the derived word, for whatever reason, replaced the PIE pater-oid

Yeah, the extra bit of detail theorized there is that atta came from children's vocabulary and the derived word, for whatever reason, replaced the PIE pater-oids. This change is carried forward in all Slavic languages. But whatever the details - the reason ojciec and vater don't sound related to you is because they aren't, they don't share a root. This does pop up even for very basic words in related languages with well-established common origins. Water, Wasser and woda sound related - the Latin aqua, not so much. It's not hard to imagine a relationship between fire, Feuer and, say, pyros (like in pyromaniac) but not so much with ogień. Then again, ogień, 'ignite' and 'agni yoga'. The reason for these seems to be that PIE had two different roots each for fire and water, respectively. Sometimes the common root lives on in fairly different words - Nebel, nebula, nebo share a root. More unexpectedly, so do guest and hostile. Or German Gift and English gift. And then you have the real winners like 'kobieta' the origin of which is essentially an unsolved mystery.