Do I need to worry if a child drinks a salt water gargle?

No. If he drank it in small amounts, it shouldn't be a problem.
Probably not. Depends on the volume and concentration of the salt water and was this just one tiem. It usually takes a child of some size to be willing and able to do salt water gargles. If one time, and not over a teaspoon of salt was used and if the child was without majory medical problems, i would not be too concerned. Otherwise, call your pcp, poison control and seek care.