Well if the eyes just opened their still wobbly because they're getting used to seeing everything but once they start playing that should be a couple weeks then you can start playing with them but if you play with them now you should be really super careful.

 Hi there,
I'd hazard a guess at about 4 or 5 weeks old. If you take them in they're still very young and will not become feral like their mum if they have positive contact with humans. In other words say hello to your new pets and enjoy them!