The majority of nurseries I have worked in serve a light tea
This is because a lot of parents like to sit and eat with their child of an evening as it's the only time they see them
Other parents don't have time due to work commitments etc
Nurseries kind of have to find a balance. Usual nursery teas to me are sandwiches, pasta salad, crumpets etc.
Although it would be ideal if those needing a full tea got a full tea and those needing just a snack got a snack I don't really know if it would work. The children would likely find it very confusing! Plus two lots of food is extra work for the staff