Frozen berries will keep for quite a while, but be aware that freezing really changes the texture of berries. They become very mushy and the juice oozes badly; frozen berries really are no good for eating out of hand unless you are willing to eat them whilst still frozen, like little popsicles.
This is especially important to think about when feeding them to toddlers. Berries are always messy, but frozen berries are exponentially messier than fresh ones. Blueberries are especially bad for staining. My DD is a berry fiend, and I do feed her frozen ones, but on occasion the blue staining on her lips from frozen blueberries has lasted for days; once I even had someone suggest that I take her to the ER because her lips were blue.