The Sport Anxiety Scale-2 has been validated in both adults and children as young as 9 years old:

http://www.pgedf.ufpr.br/ESCALA%20ANSIEDADE%20SAS%20JS%201.pdf

Additionally, below is a link to the Sport Competition Anxiety Test (MARTENS, R. et al. (1990) Competitive Anxiety in Sport. Leeds: Human Kinetic):

http://www.brianmac.co.uk/documents/scatform.pdf

It doesn't specifically state whether it is designed for children or adults, but the language used in each of the items appears to be at a junior high school level.

Thank you very much Joseph. It works very well for the dissertation project of the student of my colleague

I agree with Joseph. We have test-driven the SCAT with hundreds of children 12-18 elite junior athletes and they had no problem understanding and rating it. But watch out as this is not a real trait measure as suggested, it is also sensitive to very short duration changes (even one week).

There are several sites that you can go to:

WWW.helpinghandschildrenshospital.com or try wwwrosemond.com a family psychologist that help answer questions for parents and teachers.