> Is there a point in our life when ALL the atoms from our childhood's body gets replaced?

No. There is no point where ALL atoms have been replaced. 

This has been shown by tracing distinct carbon isotopes in DNA [which correspond reflect upon atmospheric 14C at the year of their generation (which reflect the extent of human atom bomb testing)]; Bergmann et al. 2009, Science: [Evidence for Cardiomyocyte Renewal in Humans.][1]


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for your other questions:

> Is that fact widely accepted by the scientific community?

The phrasing of Richard Dawkins, which you also list, uses "more similar" (rather than the absolute ALL). These words provide quite an accurate approximation to express that  most building blocks of cells undergo a constant renewal. The choice of words of Grand seems a bit unlucky, if read out of context.



> Are the processes that repair DNA damage operating only in damaged areas?

As a first approximation: yes. For instance the image below shows the localized detection of H2A.X histones upon damage at the DNA (red: H2A.X staining -> much stronger in treated cells which have high level of damage; blue: DNA staining)


[![enter image description here][2]][2]

> Or do they constantly "repair" also healthy areas leading to the constant replacement of the atoms that constitute the DNA?

As one approximation this would be true, e.g.: [proteostasis][3]  (a process whose failure contributes to aging).

Note that this does not exclude the possibility for rare exceptions, e.g.: [the immortal strand hypothesis][4].


  [1]: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2991140/
  [2]: https://i.sstatic.net/QxBpo.jpg
  [3]: https://en.wikipedia.org/wiki/Proteostasis
  [4]: https://en.wikipedia.org/wiki/Immortal_DNA_strand_hypothesis