In a mixed age group of children, an educator who is caring for one age range of children can also be counted against another age range of children, as long as the ratio for each age range is maintained and adequate supervision is maintained at all times.
The following article "Mixed Age Ratios In An Early Childhood Service" provides examples and information on calculating ratios in a mixed-age setting.
For more information, please read the following: Mixed Age Ratios In An Early Childhood Service
