Iron is best absorbed if the foods are eaten with food high in vitamin C, which is found in most fruits and vegetables. (blogs.uwhealth.org)
I've never made anything with frozen, but as far as I know, frozen version of most fruits sub out just fine in baking, so I think they should work. (connoisseurusveg.com)
The smallest size you'll find most fruit juices at stores are 500 ml bottles. (physiqonomics.com)