DD1 is 6 and I've been speaking to her in German since she was born. Didn't do very strict OPOL though as I found it too hard, and as a consequence she doesn't really speak any German, though she will repeat some stuff I say or throw in the odd word here and there, and generally she understands everything I say (though I do sometimes get "I don't know what you're talking about"...).
However, now that she's a bit older and in Y1, I find I'm struggling to explain more abstract or difficult concepts to her in German and often slip into English. It's partly because I can't always find the right words in German or it just sounds wrong, and partly because I'm not sure she understands what I'm saying if I use more complex words. Anyone had similar issues, and if so what did you do?