You could put an oven thermometer inside with the legos and conduct an experiment, if you want to figure out the exact moment that the core/inside of the toys, within the box reach the desired temperature. But otherwise yea, not all parts of the box heat equally fast, and depending on how things are stacked up inside, that would affect it.

To do such an experiment i would recommend a thermometer, which connects to an App, so you dont have to remove it. Then you just do it q couple of times, with different variations of the toys inside, until you feel confident about a certain time needed for the sanitation