I am afraid about my sons' spot to pimples..... :-(

[SAKRAJ]

Dear Concern,

I have three sons in total. I have twins (Named: Abrar & Sabrar), they are only 5-6 years old (younger). They had a very few birthmarks. Day-by-day as I guess those Grey little spots are increasing on whole body including face.
It was negligible for boys as they need not to be so fair; like girls.

But recently within 2/3 months, I saw some Grey spots are changing their color; like white. And then there I feel some pimples under skin. Those pimples are itch-less and painless, just under skin. A normal person can not detect this problem at the very first look. You need to take a look very closely to see marks/spots and then touch to feel pimples under skin. They are Playing, Reading, Writing and doing every thing normally. But I am very much afraid about those spots & pimples.

Please let me know, the disease type/name and what should we do? Is this a very harmful/severe disease?
Please help...

Thanks for your co-operation in advance

With Best Regards,
S.A.K. Raj
Bangladesh.