That younger sibling who shouts 'mom' when you tease them. You hit or about to him/her and he/she yells 'mom' or 'dad.'
Sounds like you're after a tattletale:
a child who tells a parent, teacher, etc., about something bad or wrong that another child has done : a child who tattles on another child - Merriam-Webster
English is not my native but a word "sissy" springs to mind, mostly for a boy behaving like that.