Perhaps some related experience will help allay some doubts and fears.
Briefly stated, our two boys attended French-speaking kindergarten and primary school in Geneva, "middle" school in England and obtained 1.9 and 2.1 Abiturs from a Gymnasium near Frankfurt a.M., Germany. One is now working in the translation and minute service at CERN, the other has a French girlfriend (French common language) and is in Chemistry research at Cardiff under a German professor.
I have no direct experience of OPOL but feel that the key is the provision of environments or situations in which languages have a dynamic life. Our principal (not exclusive) language at home was, naturally (?), English. and we took measures to maintain English learning when abroad.
We never worried when, as youngsters, the boys mixed languages or invented, especially in play, or when, later, they flipped to German between themselves. They seemed perfectly aware of what they were doing. If appropriate, corrections to inventions were supplied (just feed the real word in).
To OPOL, I would suggest, fine while only one parent present but agree on a common language between parents when together, perhaps on a day by day or meal by meal basis.
With the hindsight of our own situation, I would say that you need a positive, can do approach and the provision of support as needed, including, as the case may be, external teaching help.
Care about it like hell (they appreciate concern) but do NOT get over-anxious enough to suggest that too much is being expected. Above all, do not think there is a "perfect" approach or solution.
Language is fun, especially for the young.
I quite like de Saussure's model of the brain and language - in brief, idea processor + language coverter + speach motor. This helps pin down what is going on when we switch languages. Only a model though, not reality.
Hope this helps a little,
bon courage - und viel Spaß,
Gareth (29) & Stewart (28)
Dad Eng, Mum Anglo-Welsh
We just been around a bit.
Gareth almost regrets his Mum has no Welsh.
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