When someone is appointed as the guardian of a minor child, he or she must act in the best interests of the child. There is no way to precisely define what a child's best interests are. Basically, the idea is that the child's "best interests" are those things which will, in the long run, benefit the child to the greatest extent.
For example, a guardian must decide whether to let a minor child drop out of school when he or she reaches the legal age for that. Here is a case where the child's best interests come into play. The child may want to drop out at age 16. However, the guardian may determine that dropping out would harm the child in the long run. In that case, dropping out would not be in the child's best interests.
But there is no way to precisely spell out all things that are in a child's best interests. The guardian must act on a case by case basis as they think best.
The "best interest of the child" is a term of art. In other words it is plain English phrase that can take on a particular legal definition. It is an objective standard and not one subject to the whims of say a teen looking for money to by a Porsche so he or she can get to work. Or should I say "needs" to go to work.
Conservatorships or guardianships come in two flavors guardian of the estate and guardian of the person. Most of the time one guardian does both but it isn't necessary.
So, is the "best interest" standard expressed in a trust or will or is it in a custody context? Is it expressed as a duty upon a guardian of the person and/or estate through a decree or court order? The specifics are important because a legal standard "best interests..." are fact specific.