Toast the bread under your grill until just golden. Tear into chunks and pulse in a food processer until it's in crumbs, but not too fine. Transfer to a large bowl and stir in the parsley and the zest of ½ the lemon. Season and set aside.
Place one chicken breast in a resealable plastic bag. Using a rolling pin or something equally heavy, gently pound the chicken breast into an even thickness about ¼-inch in height. Repeat with the remaining 2 breasts. Once flattened, cut each breast into 2.
Arrange the flour on a plate and beat the eggs, pouring into another shallow dish. Coat one chicken breast in flour, shaking off any excess, then dip it into the egg, letting any excess drip off. Transfer to the breadcrumbs and coat evenly, pressing lightly to ensure the breadcrumbs stick. Place the chicken on a wire rack and repeat with the remaining chicken breasts.
In a large frying pan with high sides, add the oil. Heat until it reaches 190°C. Add two of the coated chicken breasts and fry for four minutes until golden brown on both sides, turning over a few times to prevent burning. Transfer the cooked schnitzel to a paper towel-lined plate and repeat with the remaining chicken breasts.
While the schnitzel drains, combine the yogurt, mayonnaise, vinegar and 1 tsp of lemon juice. Add the cabbage, carrot, spring onions and apple to a large bowl and add the dressing, tossing together. Plate 2 schnitzel per adult and 1 per child, serving the coleslaw alongside it.