Colette,
offering the children the opportunity to experience shadows and then to model this in the classroom with the little bears means you should not be too much at the mercy of the weather.
Carrying this out in spring or early summer adds one slight twist, given that the time of midday in Ireland once we are on daylight savings is 1.30 pm. If you have an infants class they might be going home at that time, so will have to carry out the final observation after school time.
Shadows in the winter (Sun willing) are interesting to observe since they are so long, with the Sun low in the sky even at midday.