How can i tell the parents it was an accident?

last night i babysat for a couple4 yr old daughter and 9 month old. the 4 yr old wanted to go out side but it was too hot for us all 110. she kept pouting and crying, basically she went out of side door out of the patio and proceeded to go down the stairs, apparently the neighbor saw her called the parents and i am as of now fired, ( i saw her go out and had to sercure the baby in my arms before running out side) i told them what happened but they are sticking by it. how can i tell them it was an accident?