I do not know whether it would apply to young children, but my colleagues and I developed a content analysis rubric for critical thinking that might interest you.  Our methodology is summarized in the attached resource (with citations).

Data Content Analysis of Critical Thinking

Mykolas:

I found some C.T. a few sites that may engage your interest.

 The first site lists about 10 different evaluation forms at the bottom of the page: 

The second list numerous articles on C.T.

The 3rd reads of the National Standards for Problem Solving:

The 4th is a site that offers a mini-guide in C.T. for children.

I hope that at least one of these sites helps you out. Good Luck with your search!

http://www.criticalthinking.org/pages/critical-thinking-testing-and-assessment/594

http://teaching.uncc.edu/learning-resources/articles-books/best-practice/critical-thinking#carpenter

http://ag.arizona.edu/sfcs/cyfernet/nowg/academic_component2.html

http://www.criticalthinking.org/store/products/critical-thinking-for-children/161?P=products&ItemID=161&catalogID=224&cateID=132

Here's a study that targets your age group regarding measuring critical thinking:

https://www.researchgate.net/publication/234565317_Effects_of_Teaching_Critical_Thinking_Skills_on_the_Critical_Thinking_Ability_Achievement_and_Retention_of_Social_Studies_Content_by_Fifth_and_Sixth_Graders

Best regards,

Debra

Article Effects of Teaching Critical Thinking Skills on the Critical...