I need to have a rant...

My brother brings his girlfriend's 2 kids to my house for us to mind them while shes in work and he goes the gym. The kids are okay in small doses but hes started bringing them here nearly every night. I dont want to be a cunt but jesus christ i get annoyed easily.

They have been here 10 minutes and one of my guitars have been knocked over and they thought it would be fun to play with my records..

How do i nicely tell them off without shouting....YYEEEARRGHGGG.