If one person is making 75k they aren't middle class. 75k might not go very far with kids but in that case their spouse would also likely be working so the household income would be significantly higher than that.
Edit: To be clear, the actual definition of middle class income is much lower as well:
https://www.cnbc.com/2021/07/21/middle-class-calculator.html
> That puts the base salary to be in the middle class just shy of $46,000.
I'm guessing I am getting voted down because a lot of people here think it's difficult to live on 75k, but the reality is that a huge percentage of americans are living on even less, so the difficulty of getting by with kids on 75k doesn't really affect whether an individual making 75k is considered middle class or not.