The most common and most likely answer to this crossword clue is the 14 letter word TWO-POINT-FOUR
I've seen this crossword clue in The Guardian
T
W
O
-
P
O
I
N
T
-
F
O
U
R
Buddy explains!
The straight part
We know that "averge children in the traditional family?" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case averge children in the traditional family? = two-point-four.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.