Would a disagreement between parents/guardians on a medical decision be mediated or litigated?
First, it would depend on whether the parties have joint custody or sole custody.
In a sole legal custody agreement, one parent has the unilateral right to make decisions regarding the child's medical issues.
More commonly, parties have some version of joint legal custody. With joint legal custody, the parties are required to consult with one another to allow for input from both parties before a decision is made. Both parties have the right to contact medical professionals, get a second opinion, etc. It is important to remember that it is imperative for both parties to communicate with one another respectfully and civilly during this process.
After receiving a good faith consultation, if the parties are unable to reach an agreement then they may seek mediation (which both parties must voluntarily agree to), or one party can initiate a lawsuit to go before a judge.
In either case, the recommendation of child's treating doctors is most likely to strongly influence the decision on the medical issue.