Jd fry should be large enough to eat bbs as soon as they become free swimming. they will eat infusoria too, but I'm not sure if they could go for a week on it. Stirring up the gravel around the babies can release a ton of infusoria for them to eat as well.
I have a lfs that sells live adult brine shrimp, and they live for at least an hour. I've noticed live bbs still kicking almost two hours after adding them to the tank. BTW Turning off the filter for 20min or so keeps the bbs from getting sucked up before the fry can get at them.