1-15 is the absolute maximum staffing ratio for the 3-4s but since kids come and go all the time, the ratio is usually smaller than that. Also the kids aren't in separate classrooms like most preschools so the counselors can work together and assist each other.
Each counselor on duty has a clearly defined role for each time period. Some are working directly with the kids and some are monitoring the entire area to help as needed. Another one will be the "age anchor" that checks the kids in and out to be sure that they are assigned to a counselor when they check in and that the staffing ratios are maintained.
Parents (or other guests) are always welcome to come in the clubs and observe if you are concerned.
As for the number of counselors, there are usually 9 or 10 counselors for 3-4s and 9 or 10 counselors for 5-7s and a manager and assistant manager common to both groups. They will be shown with their pictures and names on the front of the Navigators for each age group. Obviously they are not all on duty at the same time but their schedules are planned to have maximum coverage when there will be the most children in the clubs.