Scutes are made of protein so I am not sure that calcium is what they need. It is unclear what happened but if the shell is not infected it may heal by itself. You could try drying them off each day for a few hours and maybe put some betadine on the missing scute area. See how it goes but understand that healing might take months. If they are feeding fine that is a good sign. Putting them under 24 hour UVB can be dangerous. I think it would be for humans so why do it for your turtles? The other option is to take them to a vet.