Construct something which will allow each t-shirt to be passed underneath water / coloured gunge in a large cooking pot or tub. Tie a short pole across the top, slightly submerged in water, unit containing t-shirt must be passed underneath.
Best constructions will keep items clean and dry.
Time limit if appropriate with saved time at end counting in favour.
tub / large cooking pot, water / coloured liquid or gunge, various items to make safe unit out of: plastic bottles, sellotape, string etc.