Flexibly overstretched

I own three nurseries with a total of 36 staff. In the past year there have been 11 staff pregnancies. Generally the staff have returned to their jobs on slightly reduced or the same hours. However, one person asked to cut her contract from 42.5 hours a week to 16 hours. She went on maternity leave as a qualified NVQ3 keyworker and I hoped she would continue her role. But she wishes to invoke 'flexible working', which means the children in her care would see her just three mornings or afternoons a week. Legal advice has told me I must...