@kitten This is difficult to explain, I'll try, but it mostly boils down to word order and what sounds right.
Most important is that Zeit must be the last part of your word, as you're trying to describe a time, so that works.
Next part would be what kind of time you describe. So Schlafenszeit means time to sleep or time to go sleeping. Maybe other forms of this word / similar words exist (Schlafzeit maybe, but idk), but I've heard this one a lot, so it sounds right. ...
@kitten OK, so last part, it's time to sleep for kittens, so add the Kätzchen in front. Though this is a rather unusual word now, but there's no reason as to why it shouldn't work, as you're just adding two nouns together (das Kätzchen und die Schlafenszeit). Though if I wanted to express your meaning, I would've said: Es ist Schlafenszeit für Kätzchen.
But sometimes it's just more fun to go the complicated way
@Dave:blobcatgiggle: yes, I admit I worded it that way because I wanted to make a bigger word, even if it'd have been simpler to word it as a whole sentence
Ich glaube, ich verstehe es. danke schön für die Erklärung!