Emergency call boxes opening for '999' and children? (4)

I believe the answer is:

sons

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'children?' is the definition.
(son is a kind of child)

'emergency call boxes opening for' is the wordplay.
'emergency call' becomes 'sos'.
'boxes' indicates putting letters inside (the outside letters must 'box' inside letters in).
'opening for' becomes 'n' (I can't justify this - if you can you should give a lot more credence to this answer).
'sos' going around 'n' is 'SONS'.

'999 and' acts as a link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.

Can you help me to learn more?

(Other definitions for sons that I've seen before include "Male offspring" , "DH Lawrence novel: __ and Lovers" .)

I've seen this clue in The Telegraph.

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