You don't really need additional insurance coverage for a trampoline on your property however some insurance companies will not cover insurance claims related to the trampoline.
Some insurance companies will also raise your insurance costs up for your homeowners insurance if you do have a trampoline on your property.
So you may have to pay an extra $50.00 to $100.00 per month on your homeowners insurance policy to cover any accidents that may happen with other people on the trampoline.