If you, trained in CPR, have determined that beginning CPR is appropriate, you have not been able to detect signs of life in the patient (otherwise, you would not have started CPR). CPR training will have taught you how long to continue CPR in the absence of signs of life. A person trained in CPR, who determines that CPR should not be continued has not made a declaration of death, that's a different qualification, but it would seem that the patient, now victim, would then be in that place between no signs of life for too long, and legal death. As said, children are different, don't give up until the people at hospital take over from you.