Lack of proficiency in the ML due to bilingual upbringing?

Angie, without knowing too much of your situation, I think your concern is legitimate and worth thinking about. Here are some jumbled thoughts:

A couple of months ago, I was offered a place in a public ML creche, where a percentage of the children are from non-ML backgrounds but the caregivers are all native or near-native ML speakers and communicate only in high-level ML, and a private creche that is attended by children from privileged backgrounds, very few of whom speak the ML at home, and where the caregivers are near-native ML speakers who from what I have seen navigate between the ML and what for me are my mls - but not at a very high level. At the private creche, it is not rare for the children to be dealing with 3 languages at home (1 from each parent, and 1 between the parents), with the ML being an additional language. Very few of them seem to come out speaking particularly good ML.

After much toing and froing (for non-language-related reasons), and after talking to various families who had had their kids at the private creche, I decided to go for the public one for the main reason that I decided I would prefer for my son to be exposed to an excellent level of ML for several hours per day, rather than to an unsatisfactory (in my mind) mix of ML and 'substandard' ml1 and ml2. My main concern is that I want the quality of whatever language he is being spoken to in to be excellent - especially when it concerns ML and ml1. I want him to be exposed to rich and grammatically-sound language, and think (hope) that him hearing and acquiring this in the ML will help in its transfer to the mls. I believe that doing this will 'free' me in a certain sense to insist on the mls at home, whereas otherwise I would be more concerned about developing the ML too.

HOWEVER...although it is linked...I think the main issue in the case you are describing is actually parents' ambiguity about transmitting their native languages. I don't actually think that it's a problem at all for kids who are speaking one language at home to do well at school in another language BUT paradoxically, ONLY if the parents are very, very strong in consistently developing their child's ml. I think the problem tends to come, as in the situations you are describing, where the parents are perhaps not that proud of their ml, or don't think it is worth developing. So rather than teaching their child to speak the ml well, they actually speak a mishmash of ml and ML (often in the same sentence) and even 'dumb down' their conversations with the child. This then means that the child doesn't learn the basic concepts in ml that s/he should be able to transfer to the ML. Does that make sense?

To come back to my initial point, this is why I finally decided to choose the public creche. It seemed to me that the caregivers in the private one were almost too eager to speak in a mix of languages at a rather low level, rather than sticking to one and doing it well. And to add to this, many of these very privileged parents are actually quite overconfident that their children will just 'pick up' the different languages, and in many cases don't speak the ML themselves so don't realise the situation, so they don't necessarily put much thought into elaborating a strategy. I know that recent studies have shown that code-switching is ok, but I still prefer at such a young age for there to be some consistency. Of course, my children hear me speaking regularly in different languages, but it is more 'deliberate' - they know that there is a time and place for each language (and in our case, they know that I will not address them in the ML). And when I speak, I try to use a rich vocabulary. I'm confident that a word learnt in ml1 will be transferred at some point to ML and ml2.

I have many, many examples of people who did extremely well at school 'despite' speaking another language at home. But in all the cases, as far as I'm aware the parents made a strong, consistent effort to ensure an excellent ml level at home, rather than speaking in broken ML to the children or - far, far worse - not speaking to them at all.

I would echo what Adam says: if you are planning on using both German and English at home, do it, but do it well. Work out your strategy for when you will use both languages, speak clearly and beautifully, and make a strong effort to read a lot in both languages. If you are planning on just English at home, see whether you can find German support from someone else. And try to advocate in the community for parents to feel proud of their minority language!