Well, but do you discipline them at all (to be exact - how do you discipline and train them) or just expect good manners? If so, then sorry, but you might be unreasonable, because you cannot expect proper responses from an untrained horse, much less two foals. They are just being horses and they don't think about fulfilling anyone's expectations. They must have one-on-one training sessions on a regular basis and weather should not be something that can hold it back. Foals are like sponges, they suck up any information they get, in this case - that they are let to dominate a human.
Backing up after they have already done unwanted behavior will not train them to be respectful. You have to block their actions at the very second they are happening - for example, you won't see a mare backing up a disrespectful foal around, but you will see a mare blocking the foals' movements with her body language and biting, if necessary, to make him stop AT ONCE.
I have come a long way, to surrender my shadow to the shadow of my horse.