One challenge of child-computer interaction research is surveying variation of children's attitudes towards novel educational technology, which often results in an opinion ceiling effect. In this article, the authors introduce BiCo -- a bipolar continuous rating scale which builds on children's ability to draw relative comparisons. They elaborate on the development of BiCo and on how they designed it to better survey children's attitudes. Beyond addressing the opinion ceiling effect, BiCo is suitable for surveying a wide range of concepts and its invertible design enables simulation of inverted items. The authors provide data comparing BiCo with the widely used Smileyometer instrument when used by fourth-graders (around 10 years old), demonstrating that BiCo mitigates the opinion ceiling effect. They discuss BiCo as a new tool for children's technology evaluation and provide directions for future research.