Ask Mr. Smarty Plants

Your idea sounds great, but there may be a couple of problems.  First of all, how shallow is the planter box?  Tall grasses are going to need more than 6 to 8 inches of soil to grow in—something that allows around a foot of soil would be best.  Their fibrous roots can spread sideways but they also need enough soil to allow the root surfaces to be able to absorb moisture and nutrients.  The second problem is that very few grasses grow well in the shade.  There are a few that grow well in partial shade (2 to 6 hours of sun per day) and I will recommend a few of these that grow between 3 and 6 feet tall.  The good news is that you don't need to worry about toxicity with native grasses and we can avoid any with sharp leaves.  You should read one of our How to Aricles,  Container Gardening with Native Plants, that will give you good general tips about creating a container garden.

All of the grasses listed are perennials.   In the winter they will turn brown.  Some of them are attractive with their golden or brown foliage and they resprout from the roots in the spring.  You can cut the old brown foliage before or after the resprouting in the spring.

Chasmanthium latifolium (Inland sea oats) grows to 4 feet high and prefers shade.  It may require slightly more water than the other grasses listed below, but will do fine is moderately dry soil.

Sorghastrum nutans (Indiangrass) can grow as tall as 8 feet.

Panicum virgatum (Switchgrass) grows to 6 feet.

Setaria scheelei (Southwestern bristlegrass) can grow to 4 feet tall.

Here are two other suggestions that aren't grasses but of the same general size.  One of their advantages is that they are both evergreen.  Neither is toxic.

Nolina texana (Texas sacahuista) is not a true grass but a member of the Lily Family. It grows to about 3 feet high.

Sabal minor (Dwarf palmetto) is a small palm that grows 3 to 6 feet high and is evergreen.