Hi CakeBump, my husband and I were in a similar situation to yours when we were expecting DC1.
I speak the minority language (Spanish), my husband is English but also speaks Spanish fluent. We live in the UK so the community language is English.
My DH had the choice then to either speak English (his mother tongue) or Spanish (his second language). We chose the minority language at home and made Spanish the family language.
The reason we did it was because like your DP, we were worried that by using OPOL our son wouldn't have enough exposure of the minority language. Also based on my experience, most of the OPOL families I knew and even one case were they also used minority language (Spanish) at home, the children were not fluent in the minority language, and refused to use it. That's just my experience, of course, there are exceptions, as I also know one family where they use OPOL and the community language is a third language, and their child is trilingual and speaks the three languages at a very good standard.
Now, commenting on your worries, the impact of going to kindergarden and not knowing any German will depend on how old your child will be when this happens and his personality. My son went to nursery since he was 8 months old, so I didn't worry. No one expects an 8 month old to talk back to you, or to understand everything you say, actually they learn a language at this age by immersion, so you just talk to them, no matter the language.
But if your child is going to start kindergarden later at 3-5 years old? the situation will be different, but then it will largely depend on the child's personality. Some children learn the new language right away, picking it up very quickly and easily. Others might struggle a bit more, and be quiet for a while and only start talking when they feel confident enough on the second language.
In that case you just need to keep an eye on the situation and support as necessary. If the child is struggling with the second language, then you increase the support for that one at home.
When my son was 2 years old, he was a chatter box in Spanish, and also knew a few words in English. But I soon realised he wasn't saying anything at nursery as they didn't even know he could say the words in English he had been repeating at home. We then helped him a bit to acquire more vocabulary in English by getting DH to read him bedtime stories. I read bilingual books to him and told him we called this "X" in Spanish, at the nursery they call it "X". In two months he catched up with his peers, and then the teachers told me he was a chatter box in English and would not stop talking.
A comment about the child picking up mistakes from the parent that speaks the second language at home, I agree with Cory. Children are very smart and quickly realise which parent speaks at native level either language. My husband's Spanish is really good, but occasionally he might make a little mistake and my son (who is only 3) corrects him already!
And on accents, my son has my accent (I am told, by other southamerican or spanish friends). So there was no interference by DH speaking his second language. Now, DS's English is being influenced at the moment by his Spanish on the accent, and occasionally by the way he constructs a sentence we can tell he was thinking in Spanish when he said it. We expect this to correct itself when he enters school and English takes over.