There is a minhag that mishloach manos be sent via messenger and not delivered directly. I remember as a kid being driven around by the principal of our elementary school to help him deliver mishloach manos so I could act as his "shliach" for the mitzva. The Dvar Avaraham (I:13 who incidentally is not convinced there is any basis for this practice) points out that one of the achronim took issue with this practice based on the principle that a katan is pasul for shlichus. How then can a katan deliver mishloach manos as one's shliach?
The Dvar Avraham makes a brilliant distinction between the principle of "shlucho shel adam k'moso" and the general rule of shlichus. Shlichus actually accomplishes two goals: 1) an effect such as a kinyan, a delivery, etc. is accomplished by one's agent 2) that effect is ascribed to the sender as if he/she had directly performed it. The Dvar Avraham notes that for many mitzvos, as long as the first aim is achieved, the second is not necessary. When that is the case, even a katan can serve as a shliach. For example, if I were to appoint a shliach to place an eiruv techumin for me (it comes to mind because I am learning mishnayos eiruvin with my son), the Mishna says it can be done by a child. There is no halacha that the eiruv be placed by me personally - as long as it is left in the required location, however it got there, the eiruv is chal. But, when shlichus demands that I be recognized as the party performing the action through the principle of "shlucho shel adam k'moso", then a katan is pasul. For example, a katan cannot serve as a shliach for kiddushin, because it is not sufficient for the ring to be delivered to the bethrothed, but the ba'al/husband has to be the one creating the status of kiddushin (see R' Chaim stencils on this din).
With respect to mishloach manos, there is no need for the din of "shlucho shel adam k'moso" to be invoked and a katan to be excluded. As long as the mishloach manos is delivered on behalf of the sender, the first din of shlichus, this is sufficient.