I would like to begin with a correction. The Royal couple are known as HRH William , The Prince of Wales and his wife HRH Catherine, The Princess of Wales.
Onwards and upwards, the nanny employed by the couple, Maria Borrallo, trained as a Norland Nanny and is therefore highly trained to look after each individual child and their requirements of daily living, care and personal security.
Now that the children are older the Nanny will take the children to school when the parents are unable to do so and that includes collection from school and looking after them until the parents are home. I have no doubt she supervises during the time spent doing homework and outside activities. I understand that the Nanny is also teaching the children Spanish thus expanding their education.
Norland Nannies are highly sought after by Royals and the aristocracy/professionals/celebrities alike. The college was established in 1892 by Emily Ward, in Bath, Somerset, UK.