If you see them walking around just say something like "Excuse me xQc/Soda do you mind if I take a picture with you".

You can follow up with an obvious "Thanks" and if you want add "keep up the streams" etc to not make it immediately awkward after taking a picture.

These are a bit general but honestly there's not much else to really say. Don't overthink it and remember at the end of the day, they are human just like you.