My MIL has 2 Japanese Akitas and I won't let Clark stay at her house without me being there, and when we are there I make sure we are locked in the lounge! We used to live with her and both dogs have shown vicious tendancies in the past! I would definately see if the RSPCA would do anything, the only other thing you could do for you and your childrens sake is to put a higher fence up on your side just an inch or two inside the border, if she complains tell her you had no choice because she has let dogs that were originally bred to hunt bears in Japan jump into your garden!!! 
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