that logic is making me think 2+2=10, LIKE SAVE JAME AND LILY POTTER!!!!!!!!

Well the reason it worked to just go back a few hours was because it wouldn't take very long to get back to the present, but if they went all the way back to their death, that would be well more than 10 years, and it would take far too long to go back.
The reason it worked in Cursed Child was because the time turner was messed up and let them go back to the present whenever they wanted

That actually makes sense

But couldn't Dumbledore and some other people have saved Lily and James back when Harry was a baby?