Slappy birthday to you - R. L. Stine

The evil dummy comes to life to tell his own twisted tales and scary
stories!
It's bad enough Ian Barker has to spend his twelfth birthday with
his sister and their two annoying cousins. The four of them can't
ever hang out without getting into trouble. Vinnie and Jonny always
want to play with Ian's stuff. They take extra turns and break
everything they touch. Even Ian's new birthday presents are up
for grabs. But when Ian gets a Slappy dummy from his dad, things
go from bad to worse.
When Slappy's in charge, you don't fight over him, he fights over
you!