They can't find the infant as it will corrupt time now that they know she became their best mate Mels. The little girl is too tied into the events of the 60s with the Astronaut suit, changing in NYC, and possibly several other things we don't know about inbetween before she became child Mels to hang out with Rory & Amy for all those years.
Did I miss a plot point or something in the latest season of Dr. Who? Last season finale, some faction ran off with Amy's baby. Now, I get that the baby grows up to be River Song and was also Amy's best friend growing up, but shouldn't they be looking for the actual infant? However, the episodes so far seem to be more episodic than normal, with only the screen shot of the Dr.'s day of death tacked on. I guess I think it's disconnected because the through-story is a lost daughter, which is something that can't really be relegated to the back burner like "a crack in time." What do you think?