The primary purpose of this study was to develop survey questions to evaluate children’s ability to design experiments. The second purpose was to empirically examine how teaching to encourage critical thinking using Kishida and Ogura’s (2018) “watchwords for scientific experiments” affects children’s ability to plan experiments. In order to achieve these purposes, we used the experimental design confirmation sheet and experimental design worksheet we developed to teach the 6th grade elementary school science class, “Nature of Aqueous Solutions,” and also investigated the actual conditions of the students using survey questions and questionnaires. The results showed that there was no significant difference between the groups in the pre-survey question mean scores, but there was a significant difference in the post-survey question mean scores. There was also a significant increase in the mean score of the experimental design worksheet for the experimental group. In conclusion, we believe that the survey questions developed in this study can be applied to evaluate children’s ability to plan experiments. In addition, by repeating the activities of formulating an experiment plan using the experiment design confirmation sheet and experimental design worksheet created in this research, more critical thinking will be exercised to check whether the experiment plan is scientific.