It's My Party by Susan Tan

There is nothing more exciting about a birthday party than inviting all your friends to it. That is what Lynn decides to do: invite all her classmates. But there is a problem—orphans Betty and Carlos are invited, and the other children are not happy about it. The children refuse to attend the party if Betty and Carlos go. Lynn is upset with her friends’ attitude, but is determined to invite Betty and Carlos anyway. In the end, her party turns out to be something more—than she has bargained for.