Liability regarding tenant doing yard work

[Barry Wang]

I have a 3 units in Bay area. Tenants from one of the unit volunteered to lay fake grass in the backyard (500 sqft+) as long we cover all the materials.  I am cool paying for all materials, but worry about the liability if there were accidents. My tenants are early 20 young guys and had experience doing yard work before actually. 

What are my options?  I am thinking have them pay for everything and I simply reimburse them all material costs.

Do I get them sign a wavier or something? I don't know if there is any similarities, but if someone fell off when they try to paint their own room, I guess I am not liable, but this is in shared area.

I do have landlord liability in my insurance policy. 

Thanks!