But "krododili" is idomatic. It ultimately has nothing to do with "krokodiloj" in the conventional sense. However, see my other comments in this thread. You can turn a noun into a verb if the noun has an associated action.
If we understand krokodiloj to be people who speak a national language when Esperanto is more appropriate, then krokodili means simply "to do the thing that krokodiloj are known to do.
This doesn't work with knabo because there isn't just one thing that boys are known or defined as doing.
A verb generally means "to do the action associated with the root." For this to work, the root has to have an action associated with it.
- martelo (hammer) - marteli (to hammer)
"Mojosa" is a special case. It's a recently coined word designed to be extra slangy, so you can say "mojosas" as in "to give off coolness." I do not recommend overgeneralizing this "rule."
For example - rapida (fast) - rapidi (to hurry) - kuraĝa (brave) - kuraĝi (to demonstrate courage.)
"mojosi" ends with "-i". it is a verb. The part of speech can be transformed. But, roots have their intrinsic (or inherent?) part of speech. For example, pork-, is a noun root. ir- is a verb root. ven- is a verb root. But they can be transformed to other parts of speech, say, porka, iro, and veno.