Bri - Unfortunately, if you look at the NGSS (and any state standards derived from them) density is not an acceptable topic in elementary school anymore - in fact, it is EXPRESSLY forbidden here in MN. The reasoning is that density is a ratio between mass and volume, and thus is abstract and not a concrete concept. Abstract (or as Piaget called it, formal) reasoning ability doesn't start until about age 11-12, and many students in the US are not at the full formal opperations level of development until college (2/3 of students at many lower tier colleges are not where Piaget's pre-teens were 100 years ago in Switzerland, but that isn't surprising) but I digress.
You are recommending a demonstration that is actively against the standards because it is too complicated for the students to understand the science behind it. The density tower is cool. Definitely. But without the capacity to understand what is going on, what is the pedagogical purpose? It then becomes a 'magic' show.
Here is our 5th grade standard for reference - it is explicit that this should not be touched in 5th grade (or earlier).
5P.188.8.131.52 Evaluate appropriate methods and tools to identify materials based on their properties prior to investigation. (P: 3, CC: 3, CI: PS1) Examples of materials to be identified may include baking soda and other powders, metals, minerals, and liquids. Examples of properties may include color, hardness, reflectivity, electrical conductivity, ability to conduct heat, response to magnetic forces, and solubility; density is not intended as an identifiable property.