Well, to maintain that language skill you need to keep practicing. In a bilingual home it's easiest to just speak in both languages, and the kid will pick it up organically. If you're prepping your kid with flash-cards... I have misgivings.
There are cartoons made to teach languages... those I find more palatable, simply because kids seem to enjoy them. I'm not a huge fan of learning for young children that doesn't focus on 'play', and I'm wary of parents who cram babies.
That said, I would have been thrilled to have learned Greek from my father as a child, so there's certainly a wide range of possiblities. The kids I've met who seem most integrated into multiple languages have a bilingual home, the flash-card kids lose a lot of it around age 3, and generally seem less motivated to retain it.