I never used the child, as I did not have it in the beginning, where it was really good. Also the single hits take just too long for LoL. Red Lilith can wipe out everything in one hit (if your Eddie didn't do that before...).
About the heroics:
I looked into the BoS info: 1.5% for a 5* and then it's a 50:50 chance -> 0.75% for the one 5* char you want
compared to the event soul: 1.68% for a 5* and then it's a 41% chance -> 0.69% for the event char
So the difference in % isn't large. BUT you can have much more heroic souls for free compared to the event souls.