The United Nations should support a minimum age that a person can then serve in the military of recognized nations. Much like the Geneva Convention which establishes basic prisoner rights for captured people, the UN could use the same power to try and find a basic age (18) to then allow the service of those people in the military. If they do not comply, economic sanctions and possible loss of recognition by the UN are possible countermeasures.
The United Nations is kind of a joke of an organization, as nothing that they say really matters. Chances are a country that is employing children soldiers is already under massive sanctions from the United Nations that they are not listening to that setting a soldier age limite would just be one more thing that they ignored.
I think it should be up to every individual nation to decide what age a person can enlist in that Nations armed forced, the majority of nations know that child soldiers are entirely ineffective in combat and are only used in an all out type war situation like World War 1 and 2 so overall I don't think this is something the UN needs to bother with.
This would be a waste of time and effort for the United Nations. While one can argue that a certain age should be required (at least 18, if I were Grand Ruler Of Earth), the UN is essentially impotent. It can only pass a resolution. It has no enforcement powers.