"The children are playing there, where the grass is beautifully long."
Translation:A gyerekek ott játszanak, ahol szép hosszú a fű.
Oh, this sentence isn't correct. "Szép" here means just "very." The adverb would be "szépen."
I wonder whether it's equivalent to the English "nice and long", where "nice and" also means something like "... in a good way", acting adverbially though using an adjective form.
Yes, something like that.
And I guess it can switch between the adjective and adverbial forms, depending on what comes after "and", or whether the whole phrase is used as an adjective or an adverb:
nice and long - szép hosszú
nice and calm - szép nyugodt
nice and slow - this is somewhat ambiguous, isn't it? It could mean "nice and slowLY" - both "szép lassan" and "szépen lassan" sound good to me.
nice and easy - same situation, "szép(en) könnyedén".