... geht es um ein 'Bett im Kornfeld' ? ...

... we dropped down ... ? ...

Das geht so. (wie im OP, meine ich)

related discussion: sich ins Gras fallen lassen

It's not wrong in the sense of a grammatical mistake, but just thinking about what we would say, I don't think we normally use 'let' at all here. You could say 'We fell to the grass,' 'We threw ourselves (down) on the grass,' 'We collapsed on the grass,' etc. Or #1 is also good: 'We dropped (down) on the grass.'

I also don't think you need 'onto' -- but definitely 'on' rather than 'in.' Or rather, 'in(to)' only if the grass covers you up, if it's so tall that you disappear into it.

'Let ourselves fall' just doesn't sound very typical to me. You might say you let yourself drop in a context where you started out hanging by your arms, because you wanted to let yourself down slowly and reduce the distance to fall.

Just thinking about it, I think there are other languages that don't have a single verb meaning 'drop,' as in 'I dropped the glass and it broke.' In Spanish, for instance, I think that's 'dejar caer,' which is essentially 'fallen lassen.'

I think "we dropped (down) into the grass" and "we let ourselves fall into the grass" are both possible, with slightly different connotations.

"We dropped down" suggests that we were utterly exhausted and had no strength to remain upright once we reached the grass.

"We let ourselves fall" suggests that, although very tired, we still had the strength left to make a deliberate decision to collapse and let gravity take over.

... aus dem OP: ... we let ourselves fall into the grass, panting, and ...

... what made/left you 'panting' ? ...

Not into the grass unless the grass is very tall indeed, like uncut wild meadow grass of some sort.
See generally #3.

"panting" suggests we had been running, so maybe "fell panting into the grass", or "just sank, panting, into the grass"

Assuming the grass is very long, I see nothing wrong. If it isn't, then 'on to'.

Vielen Dank für all die Antworten. Ja, das Gras ist hohes Wiesengras, und da kann auch ein kleiner Bär der sich hineinfallen lässt schon drinnen verschwinden. Wer den ganzen Zusammenhang lesen will, der kann das gerne hier tun: http://www.bernibear.com/diary-of-a-juvenile-... (die Seite ist noch versteckt).

PS: Seems very fitting that ion1122 has answered here. What would you call the little stories you are writing? It is humbug, yes, but it is "intelligent humbug", if that category exists, right? Any other names for this type of literature that come to mind? :-)

Nice of you to mention my stories.

Humbug? What word would you use for that in German? Maybe Mumpitz?

I usually refer to my stories as "exercises = Übungen." (Of course, I have Chopin's études in the back of my mind ...)

Other possibilities:

fake news releases
satirical miniatures

Well, ion, I personally would never use Mumpitz, as it sounds so very, very German to my Austrian ears.

Come to think of it satire would be a broader term for what you are writing, right? My own writing is rather just humorous writing, I guess.

I personally would never use Mumpitz - ich auch nicht ... dazu kommt es mir auch arg veraltet vor ... oder es ist ein Regionalismus ... von dem ich hier im Südwesten von D mindestens genau so weit weg bin wie Sir Pooh ...
:-)

... lt. Duden und wiktionary stammt es aus der Gegend um Berlin :
http://www.duden.de/rechtschreibung/Mumpitz
Herkunft - im Berliner Börsenjargon = unsinniges Gerede; älter: Mummelputz = Vogelscheuche, (vermummte) Schreckgestalt, zu Butz 
https://de.wiktionary.org/wiki/Mumpitz
Herkunft: aus dem Berliner Börsenjargon stammend und im gesamten Ostmitteldeutschen verbreitet „unsinniges Gerede“. Dies von der älteren Zusammensetzung Mummelputz „Vogelscheuche“ für (vermummte) Schreckgestalt[1] und slawisierendem -itz (vgl. dazu auch ähnlichklingende Ortsnamen)

dazu kommt es [Mumpitz] mir auch arg veraltet vor ...

Stimmt, aber "humbug" ist auch nicht gerade Jugendslang... passt also (auch in sofern, als ich beide Wörter nicht für ions Geschichten verwenden würde).

I can't imagine anyone calling this humbug (assuming the word is currently used at all).