The most common and most likely answer to this crossword clue is the 7 plus 4 letter word BOOSTER SEAT
B
O
O
S
T
E
R
S
E
A
T
More clues leading to the result BOOSTER SEAT
Buddy explains!
The straight part
We know that "thing for child in car" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case thing for child in car = booster seat.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.