I had never heard of this before until I read your question and researched a little bit. As far as I have learnt, the greyish spots I don't think are anything dangerous and just have different pigmentation from normal.
This is a quote from a pediatric opthalmologist on a site where someone was worried about the same 'problem':
"It could be either of two things. The layer under the sclera (the white part) is called the choroid, and is very darkly pigmented... So if the sclera is slightly thinned that area can appear darker.
Or it could be a normal variant pigmentation called ocular melanosis, where there is extra pigment in the sclera and this causes the sclera to appear slate grey."
Just to make sure, it would probably be best to arrange a quick check-up at the doctors for your child, in case it is anything more severe.
This is a quote from a pediatric opthalmologist on a site where someone was worried about the same 'problem':
"It could be either of two things. The layer under the sclera (the white part) is called the choroid, and is very darkly pigmented... So if the sclera is slightly thinned that area can appear darker.
Or it could be a normal variant pigmentation called ocular melanosis, where there is extra pigment in the sclera and this causes the sclera to appear slate grey."
Just to make sure, it would probably be best to arrange a quick check-up at the doctors for your child, in case it is anything more severe.