The most common and most likely answer to this crossword clue is the 6 letter word BOOING
I've seen this crossword clue in Financial Times
B
O
O
I
N
G
Buddy explains!
The straight part
We know that "Kids’ reaction to “Time for Bed”?" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case kids’ reaction to “time for bed”? = booing.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.
What does BOOING mean?
BOOING verb- To shout extended boos derisively.
- To shout extended boos at, as a form of derision.
- A disapproving exclamation by a member of an audience.