I think yes data can be collected from 8-12 year old students if there are no ethical issues. In this age students are well aware of most of things. You should pretest the questionnaire by collecting data from students having different iq level then may make necessary changes if required.
Ain - I think that there is no problem in using a questionnaire among 8-12 years old students but it is better to use interview schedule rather than questionnaire to get information from 8-12 years old students. The interview schedule has the advantage that where students find difficulty in understanding the questions the researcher or the surveyor can explain these questions to them in detail and also can repeat the same. The need is to make your interview schedule simple so that the students can understand it. To get sensitive information, ethical consideration is also needed.