A 2.5-year-old child is ill for the second day. The onset of the disease was associated with the temperature up to 37.8°C, a single bout of vomiting, and watery diarrhea up to 5 times per day. During the second day, vomiting occurred twice, body temperature is 38.0°C, the child has low appetite, watery diarrhea continues. The treatment of the child should start with the following: