“as” clause and tense

The simile refers to things that children presently are in the habit of doing, independent of the tense of the main verbal clauses.
Simpler one: "She hugged her son, as mothers do." It would be weird to say "as mothers did." We would have to ask why mothers no longer give hugs.
Also, the different tenses in the clause could be rendered ". . .as a child, having cried itself to sleep, (then) continues to sob. . ."

I agree with Markustenhaafus that the simile refers to general time and doesn't need to agree with the tenses in the main clause, but in that case why use past perfect rather than present perfect in the 'as' clause? That's the strange one for me. Why not simply - 'as a child who has cried itself to sleep continues to sob in its dreams.'?

Yes, I thought that it had been used because it was a factual statement but was not sure, and because I rarely see these types of sentences, I thought it would be best to have second opinion.

Will, I think that the pluperfect "had fallen asleep" demonstrates separation from present state. Kind of disjunctive, or even concessive.
Present perfect can infer past action with an effect on the current state.
Pluperfect or past perfect means, as I understand anyway, totally completed, done.
Present perfect: "she has fallen asleep and therefore, she now snores."
Pluperfect: "she had fallen asleep, yet she continues to sob."
If the tense is deliberate, I think this could be the reason.

@Markustenhaafus - You could be right, but in the indicative past perfect (or pluperfect) is usually used in conjunction with past simple or past continuous, as it signifies a 'further past'. I agree past tenses can be used for distancing - 'How long did you want to stay for', 'Were you wanting something to eat', but when we use past perfect for distancing, we usually still use it in conjunction with a verb in past simple or continuous. 'I was wondering if you had decided what you want to do'.

Of course it's also used (as a hidden subjunctive if you like) in past counterfactual conditionals and related forms which use the 'Unreal past', such as 'I wish I hadn't said that', but that's not what we have here.

Fairly recently I wrote a post for my blog on the twelve tense system, for which I did quite a bit of research on verb forms, and I can't remember seeing past perfect being used in this way.

I don't think it matters too much whether 'continue' is in the past or the present, but I think that after 'as a child who had cried itself to sleep', I would expect something like 'would continue to sob in its dreams'.

Incidentally, I too was brought up on 'pluperfect' and plain 'perfect', but now I teach foreign students I've learnt to use the standard EFL expressions - 'past perfect' and 'present perfect'. I didn't like it at first, but now it makes sense to me: four past tenses (or forms, if you prefer), all starting with the word 'past'. Actually, I quite like the French name 'plus-que-parfait', which suggests this aspect (no pun intended) of going back.