After you sign a contract, you will be violating terms of contract and would attract penal provisions of contract. and conduct & discipline in your employment. When your father does not know abc of s/w, any time you could be caught doing benami business through some of your jealous colleagues. If you find the clauses of contract with the company, you can find stringent conditions in the language of the contract agreement. Moreover, while in job, software engineers have to devote long hours of duty (say 12-14 hours frequently) on the time bound projects. So, your part time business will definitely will get badly hurt keeping your clients dissatisfied while your attention would remain divided on employment and business. So, you would neither be able to devote your attention properly to your official duties, nor to your part time busuness, even if your business goes unnoticed for most of the time.
But, question arises, if you are earning a handsome income through your own business, my advice is to concentrate on your own business rather than joining a job.
However, if you are able to get very attractive job with good career prospects, you should start shedding your part time business by and by.