Could this function as a sort of modal verb, e.g. could one say something like "Biarkan mereka lihat" for "Let them see"?
So, if I am understanding that dictionary entry through the fog of Google Translate, it's a sort of particle indicating a result. Is the -kan suffix making it a kind of imperative verb, or does it still have the sense of a particle here?
It's more like this :
"biar" (particle) + me-kan (affix) ==> benefactive verb.
Because the verb is used as an imperative, the me- is dropped.
Saya membiarkan mereka menonton TV.
(I let the watch TV)
Biarkan mereka menonton TV!
(Let them watch TV!).