how can i keep the neighbourhood cats out of my garden?

i do not want to hurt them, i use to like them coming in, it never bothered me, but now my 11 month old son plays out in the garden, but i cant let him out there no more as the cats are crapping all over the place (they never use to do this) and it really stinks sometimes, how can i keep them away from my sons play area.

thank you
 
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