Thai does not have upper case letters.

It does have a character that essentially means duplicate the previous word which is a common way to pluralize or emphasize:

Child = เด็ก “dèk”
Children = เด็กเด็ก or เด็กๆ “dèkdèk”

Pretty = งาม “ngaam”
Pretty! = งามงาม or งามๆ “ngaamngaam”