The translation is quite literal, I believe

"to" seems to be correct (google check, my memory was somewhat faulty)

Leider weiß ich nicht, ob die Sache mit dem Vater des Gedankens englisch idiomatisch ist.

"If we want this be not based on wishful thinking only we have to consider carefully with what our patients can cope."

"Was kann der Maßstab der Bewältigbarkeit sein" bedeutet wahrscheinlich nur "was können unsere Patienten bewältigen". Das kommt wahrscheinlich von einem verhinderten Schriftsteller, den der schiere Hunger auf eine Karriere als Psychiater gedrängt hat.

könnte es sein, dass man mit wishful thinking an die sache rankommt?
sehe gerade, andreass sagt das auch schon.
also tschüss!

i agree in this sentence it amounts to wishful thinking

"Wenn wir wollen, dass hier nicht bloß der Wunsch Vater des Gedanken ist, dann müssen wir genau überlegen, was der Maßstab der Bewältigbarkeit seitens unserer Patienten sein kann."

if we want to go beyond wishful thinking, we have to examine carefully to which extent our patients are able to accomplish this.

'Thy wish was father, Harry, to that thought.'
—Shakespeare, Henry IV Pt. II (IV.v.91)

web hits:
the wish is father to the thought - 646
the wish is the father to the thought - 352
the wish is the father of the thought - 70
the wish is father of the thought - 18

The saying is evidently one of the many in English that come from Shakespeare, even if most of us are no longer aware of the connection. Anything similar would be recognizable, but I would recommend the version above, which is both closest to the original and most common.

As for the actual example sentence, though, I tend to agree that the quote is out of place. Off the top of my head, I would say that 'The wish is father to the thought' means that you only have to think of something to start wishing for it; that as soon as you realize something is possible, especially if it's something bad, you're tempted by it, you want it to be real.

So I agree with the majority that the writer seems to have actually meant something more like Wünschdenken/wishful thinking. I would translate very loosely along these lines:

If we do not wish to merely indulge in wishful thinking, we must consider very carefully just how much a patient can be expected to work through / overcome / get over / deal with / cope with / etc.

I would correct the proposed translations as follows:

@AndreasS: —> If we do not want this to be based on wishful thinking alone, we have to consider carefully what our patients can cope with.

@noli: —> If we want to go beyond wishful thinking, we have to examine carefully to what extent our patients are able to accomplish this.

@hm-us: I know it's not the crossover chat but I hope you don't mind:
Wunschdenken (not 'Wünschdenken')

Oops, thank you. Umlauts are one of my nemeses. (-;