Going completely off the wall here, but the Palladium system has a mechanic for that. It is a spell point system, to start with.
Each person has a specific amount of spell points, (called PPE, IIRC), which most don't ever learn to use specifically, save for spellcasters.
At the moment of death, that PPE is doubled, then released, and can be used to power spells, create items, recharge your personal power, and so on.
Children have lots more than most adults, who tend to burn some of their magical potential and convert it into talent and skills and such, around puberty.
Thus, sacrifices can gain you power, and sacrificing children more power. Of course, sacrificing sentient beings, particularly innocent childrens, is an EVIL act, with all caps, but some people will go there for the power it brings.
Not very useful without a lot of conversion work, but there it is.