I think the only moral choice is to try to save the child. If you didn’t, to me that is the same as seeing a child being forcefully abducted today, and doing nothing.
The time/space continuum, the butterfly effect, causality, or what have you may strive to trip you up, but that has nothing to do with your choice of what you do.
You have to try.
I saw a boy drown on a beach when I was thirteen. I watched while I flew a kite. A few people tried to save him, but most people including myself just stood and watched. I felt loathing for those people (including myself,) who stood and watched.
It’s like those people from the Titanic in the half-filled lifeboats that didn’t go back for the people crying for help in the water. I don’t know why we, as a group, don’t help each other.
I would want somebody to help my family if they were in severe distress. Part of the social contract, in my opinion, means that you have to be willing to do the same. More, you have to be prepared and capable to whatever extent you can.
So, I’ve told myself that if I hear somebody crying for help or see somebody in distress, I’m not going to stand and watch. I’ve never been in a situation where I’ve had to put myself at risk (like running into a burning building, or interrupting a rapist,) so I don’t know if I would have the guts to follow through on my conviction. I hope I do. I hope I never find out.
I think you have to try. I think it’s the only moral thing. I can’t hold much respect for someone who would sit idle while a child was decapitated, even if they believed their failure was preordained. You have to try.
On another note, if you don’t try and at least tell the police, you have knowledge of the incident that may make you an accessory before the fact.