A possessor of land is subject to liability for physical harm to children trespassing thereon caused by an artificial condition upon the land if: (a) the place where the condition exists is one upon which the possessor knows or has reason to know that children are likely to trespass, and (b) the condition is one of which the possessor knows or has reason to know and which he realizes or should realize will involve an unreasonable risk of death or serious bodily harm to such children, and (c) the children because of their youth do not discover the condition or realize the risk involved in intermeddling with it or in coming within the area made dangerous by it, and (d) the utility to the possessor of maintaining the condition and the burden of eliminating the danger are slight as compared with the risk to children involved, and (e) the possessor fails to exercise reasonable care to eliminate the danger or otherwise to protect the children.