can it fit a 11/12 year old child? How small can it go?

I'd say it may be too large for 12 Y/O: I suggest you try "OXFORD ULTRA THIN MOTORCYCLE BALACLAVA MOTORBIKE UNDER HELMET THERMAL WINTER BLACK" (from J&S / Amazon) This would most likely be ok, it is smaller than an Adult fit for def, it was too tight for me...worth a go.

Is this really thin as got tight helmet. Want to use in the summer as well is it to warm for this

It is really thin and it is fit for summer as well. Silk will make you feel cool in summer and warm in winter.

Has anyone used this under a full-face motorcycle helmet? If so, does the fact that there is no opening for the mouth have an impact?

I use it all the time in winter under my helmet,and I would recommend it,i bought two different ones but would recommend the jasmine pure silk x

Would this fit a 6 year old?

Hi, I am a 26 year old male with a fairly small head ( I wear small and extra small motorcycle helmets) and it is slightly loose on me. I would say it will be quite large on a 6 year old.