How to I add my kid salary as expenses on my business

In the Business Info section, you will want to indicate that you had employees.  You will then proceed through the interview and can enter the wages.  

See Family Help:

If the business is a parent’s sole proprietorship or a partnership in which each partner is a parent of the child:

1. Payments for the services of a child are subject to income tax withholding regardless of age.
    
2. Payments for the services of a child under age 18 are not subject to social security and Medicare taxes.  If the child is 18 years or older, then payments for the services of a child are subject to social security and Medicare taxes. 
    
3. Payments for the services of a child under age 21 are not subject to Federal Unemployment Act (FUTA) tax.  If the child is 21 years or older, then payments for the services of a child are subject to FUTA taxes.