We utilized the new high-order (250-378 mode)Magellan Adaptive Optics system (MagAO) to obtain very high spatial resolution observations in "visible light" with MagAO's VisAO CCD camera. In the good-median seeing conditions of Magellan (0.5 - 0.7″) we find MagAO delivers individual short exposure images as good as 19 mas optical resolution. Due to telescope vibrations, long exposure (60s) r' (0.63μm) images are slightly coarser at FWHM=23-29 mas (Strehl ~ 28%) with bright (R < 9 mag) guide stars. These are the highest resolution filled-aperture images published to date. Images of the young (~ 1 Myr) Orion Trapezium θ1 Ori A, B, and C cluster members were obtained with VisAO. In particular, the 32 mas binary θ1 Ori C1C2 was easily resolved in non-interferometric images for the first time. Relative positions of the bright trapezium binary stars were measured with ~ 0.6 - 5 mas accuracy. We now are sensitive to relative proper motions of just ~ 0.2 mas/yr (~ 0.4 km/s at 414 pc) - this is a ~ 2 - 10× improvement in orbital velocity accuracy compared to previous efforts. For the first time, we see clear motion of the barycenter of θ1 Ori B2B3 about θ1 Ori B1. All five members of the θ1 Ori B system appear likely a gravitationally bound "mini-cluster", but we find that not all the orbits can be both circular and co-planar. The lowest mass member of the θ1 Ori B system (B4; mass ~ 0.2Msun) has a very clearly detected motion (at 4.1 ± 1.3 km/s; correlation=99.9%) w.r.t B1. Previous work has suggested that B4 and B3 are on long-term unstable orbits and will be ejected from this "mini-cluster". However, our new "baseline" model of the θ1 Ori B system suggests a more hierarchical system than previously thought, and so the ejection of B4 may not occur for many orbits, and B3 may be stable against ejection long-term. This "ejection" process of the lowest mass member of a "mini-cluster" could play a major role in the formation of low mass stars and brown dwarfs.