What you describe sounds like Baby Bottle Decay. Do you ever put the child to sleep with the bottle? Do you ever let the child just hold on to the bottle even though it's not feeding time? The tongue protects the lower teeth from being bathed by the juice/milk. This is a specific pattern of decay. Here are some easy websites to get more info and see pictures:
Remember, a child should NEVER be put to bed with a bottle. Too mamy parents use a bottle as a pacifier. There is no amount of tooth brushing that can compete with a Sugar bath that lasts all night.