If your child is feeling particularly anxious about returning to school. Maybe you could create a 'hug button'. You could use two old buttons, little hearts or special beads. Keep one for yourself and either sew the other one onto your child's top label or sleeve, or attach it to hair bobble to wear around their wrist or perhaps attach to a scrap piece of material to keep in a pocket.
Explain to your child that whenever they're missing 'mummy/daddy' they can press the hug button and you will send them a hug and be thinking of them.