If you share a yard, you may be sharing liability with your neighbor and the landlord. Not to mention that the landlord should know that he now has an "attractive nuisance" on his property. What if he finds out on his own and asks why you didn't inform him? What if your neighbor decides to lie and say that you put up the playset? What if a child goes onto the playset unsupervised and injures him/herself - would you be able to face that child's parents, knowing that with a phone call to the landlord, you could have prevented that (hypothetical) injury?