The while-employed restrictions may well be enforceable.
Think about an accounting firm or a law firm. Accounting firms do not want their salaried employees moonlighting as tax preparers or consultants for other firms. Law firms don’t want their associates taking on clients and cases that may well conflict with the firm’s clients and cases.
It is reasonable for one’s full-time employer to expect all work efforts in that field to benefit that full-time employer.
But post employment restrictions need limits. Limits are often expressed in terms of time and/or distance.
Employment contracts should be fully understood by all parties to the contract. In this case, you are being told that the employer doesn’t understand the legal stuff, but that your son or daughter should sign it because all of the other employees did.
P.S. Worth noting: The last line of the California law cited in post #1 in this thread places the burden of proof upon the employee. This is not good for any employee who signs such an employment contract as being offered to your son or daughter.