just had a sort out in my phen and loads of stuff has got furry mould growing on it, including the kiddie backpack. The shed is bone dry with plenty of sun.

Lack of ventilation?

Its probably damp air, turning into warm damp air in the sun.

no ventiliation seconded

lack of ventilation every time.

Drill some 1 inch holes around the base of the shed and just below the eaves, then staple some green garden netting over the holes to stop imigrant incects invading.