Which oxygen delivery system most reliably delivers a high (90% or greater) concentration of inspired oxygen to a 7yo child?

Nonrebreathing face mask.

Explanation:

The oxygen is one of the most important component of the atmosphere as well as important for the body as well. The living organism inhale oxygen and exhale carbon dioxide.

Non breathing face mask is a type of oxygen delivery system used in case of the oxygen therapy given to the patients. The individuals suffering from chronic respiratory disease and the individual unable to inhale oxygen requires this mask.

Thus, the answer is non breathing face mask.