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Two of my work colleagues have a child at the Buttercup Montessori on Watchouse Lane, and are very positive about it.
One of my friends has his youngest at the Buttercup Montessori in Broomfield. That is apparently good too. He previously had his children at the Elim Day nursery, but moved them because the Buttercup had a better educational setting.
I've met a few people with children at the Rainbow Pre-school in Chelmer Village. That is a good pre-school, nothing special educationally.