The Sea Castle by Susan & Wendy Mitchell

It is a very hot day at the beach. Zoe has a brand-new bucket and spade which she cannot wait to use. The sand is too dry to build a sandcastle with, she needs to wet it. The sea is too far out, she is not allowed to go down to it alone. Zoe tries waiting patiently for dad to take her, but it is not easy. That is, until something quite amazing appears in the sand beside her. Zoe watches as the creatures of the seaside work together to help her solve her problem, building something more marvellous than she could ever have imagined. The strange thing is this special, natural sort of magic just seems to vanish – poof! – the moment a grown-up lays eyes on it…