The child already died in the accident. The point was, it's not wise to use powers imposed by another being to reverse the natural course of events.
Because as we all know from history, using powers imposed on us to reverse the natural course of events leads to 50,000 deaths every year. Even today's scientists are still dumbfounded by this phenomenon even to this day....
Wait, what am I talking about? That's never happened. And in the field of medicine, we always try to reverse the natural course of events. That's what medicine is about. Oh, you're dying of cancer? That's natural. Aids? Natural. Blood loss? Natural. Death? Natural. If we had the technology to bring someone back to life shortly after they died, you think we'd use it or just go "Nope! We are not wise enough to use such technology!". And if you're going to go the "We didn't create it" card, don't. We are using technology to do incredible things that we as individuals didn't think of ourselves. Like me on my computer. I didn't create it, build it, or invent it, yet I still use it because it's a great tool. And this is a tool that was given to me as a gift. Riker himself was given a gift, but decided not to keep it for no discernible reason.
Picard: Power corrupts.
Riker: And absolute power corrupts absolutely.
Ok, so what do you call this Galaxy Class Starship that has the power to beam people in and out of existence, re-create any object, setting or life form from any planet, ship or station from any time, create it's own life form, and pulverize an entire solar system with it's heavy armaments? Having that power is ok, but having the power to bring a dead girl back to life? You're worse than Satan.
Honestly, put yourself in Riker's position. What is the downside? What could possible go wrong that it is better to leave a dead girl behind rather than bringing her back? Is it that this girl might cause an event that will wipe out all life in the galaxy? Well, if you're going to go by that frame of logic, you can pretty much use that philosophy on EVERYTHING. If I wear socks, will I get fired? If I get a pet, will the space station explode?
But here's the big one. And this is a really, really, REALLY big one.
Neelix died in a Voyager episode. Seriously. He was pronounced
DEAD. However, 7 of 9 used her borg technology to bring him back. You know Borg technology, right? Those powers had to be imposed on her, and the crew didn't have anything to match it. but instead of getting into an episode long debate on the moral of using this power to reverse the natural course of events, they just decide to do it. And guess what happened? Everyone ended up fine in the end. Whatever troubles that this event caused, the characters worked it out with each other.
Yep. We cannot use powers beyond our understanding unless it involves saving Neelix.