These are two different kinds of adjectives, the first is called predicative adjective (always comes after the noun or another word) and we have to pluralize that. Examples: A lányok szépek. or A gyerekek kedvesek.

However, if the adjective comes before the noun (attributive adjective), it remains singular, just like in English. Examples: "Szép lányok." "Kedves gyerekek."

I would like to add one remark to what Fruzsina said:

As far as I see it is exactly the other way round as in German:

Az ablakok különösen szépek. - Die Fenster sind besonders schön.

Ott vannak a nagy múzeumok. - Da sind die großen Museen.

In other words, in German, in a construct like "<noun> is/are <adjective>", you don't have to match the adjective with the noun. In a construct like "[the/an...] <adjective> <noun>", you do have to match the adjective with the noun.

In Hungarian, it is the opposed. If you have "<noun> is/are <adjective>", then you have to match them. In a construct like "<adjective> <noun>", you don't have to match them.

There is one more twist in Hungarian, though: in third person (singular and plural), the is/are verb is omitted, in constructs like "<noun> is/are <adjective>". For instance:

The window is beautiful - Das Fenster ist schön -- Az ablak szép. (Literally: "The window beautiful." "Das Fenster schön.")

The windows are beautiful - Die Fenster sind schön -- Az ablakok szépek. (Literally: "The windows beautiful". "Die Fenster schön.")

(This is NOT true for first and second person though: "You are tall. Du bist groß. (Te) magas vagy." "We are rich. Wir sind reich. (Mi) gazdagok vagyunk.", ... etc. But notice that in plural, we did pluralize the adjective too: gazdagok)