PHOENIX — Arguing that medical marijuana has been the most effective treatment for their son’s seizure disorder, the parents of a 5-year-old boy filed a lawsuit here on Monday to force state officials to include marijuana extracts — oil-like resins with very low levels of the psychoactive ingredient THC — as a legal product under the state’s medical marijuana act.

As it stands, the act, approved by voters in 2010, allows patients to use “any mixture or preparation” made with dried marijuana flowers, like brownies. The boy’s parents, Jacob and Jennifer Welton, have been crushing the flowers and mixing them into applesauce, which they say has become difficult for the boy to ingest after brain surgery last year compromised his ability to eat. They do not want to buy the extract, found on the black market, for fear of being arrested.

“We’re not criminals,” Ms. Welton, 30, an enrollment adviser at the University of Phoenix, said in an interview. “We just want what’s best for our son.”

The Weltons’ legal action opens a new front on the fight over legalizing marijuana for medicinal use across the country, focusing on very sick children to highlight its potential benefits.