My child only eats milk and sweets, what do you advise me? (3 years and 4 months?

Kids at 3 are not at an age where reasoning works. Don't try to reason with her. If you only offer healthy food & several tasty options, you have done your job. If she skips a few meals she may be willing to try things others seem to enjoy. Bribery is fine for trying new. As long as you give only milk & sweets for every meal she will demand you continue.