Shattered patio glass pane

Other

Hi,
My tenant phoned me yesterday that my patio door glass was shattered. He alleges that his wife saw the shattered glass after she dropped off the kids to school and returned to the property.
Knowing that he has three (mischievous) children I asked him how it happened. He said he didn't know but said maybe because of the weather and reminded me of the somewhat wet and windy weather over the last few days to which I said glass just doesn't shatter because of that.
So I went to the property to have a look at could see that there was a point on the outside (garden facing) of the patio glass door which had an indentation where someone has probably thrown a stone and from which point the glass shattered. I pointed out to the tenant that it is possible that his kids (who have caused other damage in the property in the past) could be playing in the garden and could have been playing with small pebbles in the garden and playingly thrown one towards the glass patio door. He denies this.
Questions I have include 1. What should I do in this circumstance. 2. Whose responsibility is it now to fix the large panel of glass on what is likely to be an expensive job 3. I am not sure if the replacement of the glass panel will be covered by my buildings insurance. Even if it is, there will be an excess to pay and my premium next year is likely to go up because of the claim.
Any advice and guidance would be appreciated please.
Thanks