I think I would be inclined to move.
Not in a rush panic, but start looking and certainly dont link it to this awful incident.
For me as a mum of 2 DDs I would be as gutted as you are if something like this happend.
You are fortunate in that you dont own your home and so moving is not as difficult as it could be.
I would have 2 many ifs,buts and maybes going around in my head to stay there and know that the way I would be with letting DD out to play (even in the garden) would be very hard to do and ultimately by staying put - worry that my stresses would pass onto DD. Another thing I would be concerned about is the ongong relationship with the landlord - the boys Grandfather. With due respect to him and the family, they may well be happy to keep hom away at the moment but this is all very fresh and new. Everyone is probably reeling in someway from the shock and fall out from this, including your landlord. However, as time goes by and authorities involvement lessens or any potential charges are brought forward the relationship between you and your landlord could change or they may eventually start letting the boy come back to the house, and perhaps without even notifying you.
Also, another thing on the back of my mind is that some behaviours are picked up from other family members. This is not always the case but can be.
For me there would be too many doubts and worries playing on my mind. The fact that the landlord is the boys Grandfather makes it possible for repercussions of this event to be thrown up at any point in the future. If you move - that risk is gone.
Sorry to be so doom and gloom. I hope I have not added to your worries. I expect you have run through all possible scenarios anyway in your own mind.
Like I said, if you do decide to move, it does not have to be done in a rush to somewhere sub standard etc but I would certainly suggest starting to look around.
Good luck and I send my best wishes to you, your DD and your family.