New neighbours have a dog who can jump the fence, the fence is my responsibility - Who should pay?

So I have new neighbours who have a dog who can jump the fence between our houses. The fence is not very tall, maybe 4ft / 1.2m and is my responsibility to change, should it fall / break.

I don't want their dog in our garden as we have a 15month old son who will be playing in the garden when the weather is nicer. Who should pay for this?