In addition of assuming that minimum wage is directly correlated with unemployment, which it isn't, there's another hidden assumption here.
The hidden assumption is that lower-paid positions are less important. They aren't. There are many minimum wage positions that are more important than higher wage positions.
They're not lower paid because they're less important, they're lower paid because they're more replaceable.
So, when you force the employer to choose between not having a janitor, or paying 3.5$ more per hour, they will certainly decide to pay 3.5$ more per hour.
Assuming that there is profit, which is a good assumption as the average surplus value rate is of around 5-7%, then what will happen is that profit will decrease, and employment will stay more or less the same.
It's very important to realize that the market values replaceability, not importance.