Another cultural Orthodox here. I too was allowed beer with lemonade or watered down wine as a kid. I was also told I couldn’t be a sinner as a kid, because I didn’t have the capacity of mature judgement yet. (The legal system acts in a similar way.)
I presume your parents are just using sin to discourage you without having to explain themselves. Greeks in their position might say krima “shame”, maybe even krima apo to Theo “shame according to God”. But I’d be surprised if they used the term amartia “sin”.