Can I be convicted of criminally negligent homicide if I fail to get medical attention for someone who needs it?

Maybe. If you owe a duty of care to a person, then you are required to act without criminal negligence. A duty of care may be imposed in many ways including a familial relationship like caring for a child, or employment like a lifeguard. It can also be imposed if you accept a duty. In other words, if you choose to try and aid or help someone, you must act in a way that is not criminally negligent.