TOPIC:

Yes, you can always use multiple confirmation points in your questionnaire to filter out children. But I think if its serious research that you plan to conduct, its better to have a proper participant recruitment plan. Later you can ask the selected participants to fill a questionnaire online. In this way, you have a greater idea and control about your sample.

And also you can be cautious where you look for participants. Maybe you can avoid looking for participants in places where minors hang out. For example social networking platforms etc.

Well, to be honest, there are no exact method to filter them. I know this is a tricky task for a researcher. There is always a chance that a child can pretend to be an adult in an online setting.

What you can do is when you develop your questionnaire, you can include multiple confirmation points (asking for age, later asking for year of birth, etc) in your questionnaire to verify the age of the participants.

I have recruited participants for a research study online. How do I know that there are no children amongst the selected participants?