This resource appears to be designed to build towards this disciplinary core idea, though the resource developer has not explicitly stated so.
Comments about Including the Disciplinary Core Idea
It will be helpful to make explicit the connection between the models representing the atoms involved and the Law of Conservation of Mass. Depending on the level of your students, you may also want to include the relationships among these two variables and the balanced equations. In other words, the number and type of atoms in the reactants and in the products must be made equal because matter can be neither created nor destroyed. Once the idea that the number and types of atoms before and after a reaction must remain constant, it is straightforward to establish that the mass before and after the reaction also must be constant. One could use atomic masses from the periodic table to actually calculate the masses for each atom or molecule on the reactant side and then for the product side to illustrate this point, though this is beyond the assessment boundary for middle school.
An interesting thing to note is that the simulation will differentiate between a balanced equation and one that is simplified. In other words, a student will be given feedback that his or her equation is correctly balanced but not simplified if the answer 4HCl = 2H2 + 2Cl2 is submitted. However, if an equation is balanced and simplified, the feedback is simply that the answer is correct, not that it is both balanced and simplified. It will be helpful to point out to students that we use the smallest whole number ratios of coefficients to balance an equation. Students may not even realize that this is the case until they encounter a discrepant event.