Gun-related deaths, drowning, suffocation are next on the list at http://www.childdeathreview.org/nationalchildmortalitydata.h... . However, motor injury outnumbers all of the other listed fatalities due to unintentional injury.

A lot of the numbers in that list are due to infants <1 year old. The CDC more generically says "malignant neoplasms" - cancer - is about 55% as likely as as unintentional injury for the ages of the children in the report. http://www.cdc.gov/injury/images/lc-charts/leading_causes_of...

To answer your question, gun-related deaths and cancer.