As Lydia says, it's the really old, basic words that tend to be weird (they've had more time to get mucked up), and as gd says they often come in via German. Two of your examples have dropped from PIE (relatively) unchanged: '*uks-en' and '*muHs-'.
'*ped-' has been on a bit of a journey to become 'foot' and '*dont-'/'*dent-' to become 'tooth' (but seem to have survived into Latin OK) and PIE 'gu̯elbh-' -> PGm '*kiltham' -> OE 'cild' -> 'child' is sort of visible if you squint.
What's funky about 'child' is that the OE 'cild' was both the singular and the plural (a bit like sheep), but it was then re-pluralised in the 12th century: The resulting 'children' looks more like the PGm '*kiltham'. Prediction for the year 10,000: Children we be called 'gelfs' as we regress to PIE. 
As an aside, I-mutation is common in these old words from their passage through German.
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