do I get retirement and disability benefits regardless of work and income in the years after I earned the 40 credits?
For retirement and survivor's benefits, yes. For disability benefits, you must have earned 20 of the credits in the 10 years just prior to your disability, unless your disability is blindness. Source
However, note that the amount you receive can go up with additional years of work, since it's based on the average of the top 35 years of income. If you are under 40, it is near certain that many of those years of income are going to be zero, bringing down the average. (Credit to Dilip Sarwate)
Also, in the Social Security statement, it says that the spouse and child are eligible for a certain amount per month; is it per child?
Without seeing the statement you are referring to specifically, it is likely per child. However, note the family maximum.
Because it also says "maximum benefit per family" has an X amount of monthly benefit. Does that amount include myself?
Again, without seeing the statement, the answer is likely yes. Source
The total depends on your benefit amount and the number of family members who also qualify on your record. The total varies, but generally the total amount you and your family can receive is about 150 to 180 percent of your full retirement benefit.
