Do you know the name of the company with whom your mother took out the insurance policy? If so, you should write to them quoting the policy number and can then find out whether it was actually in your daughter's name.
Also, did your mother make a Will and was your father the Executor of it? As executor he would naturally have been the one to handle the tidying up of her affairs, but if indeed the insurance policy was in your daughter's name then he has no legal right to keep the money and the insurance company might possible make out the cheque in your daughter's name, although it could be made out in your father's name as Executor. However, if the policy was in your mother's name, your father has a legal right to it, but not a moral one if he knew your mother intended the money for her granddaughter. if you can't persuade your father to hand over the money, you will have to decide whether �1500 is worth causing a family rift over.