Great, you made some Dutch friends. You are even invited by a friend to a birthday party! And they don't just invite anyone, so you can feel honored. But wait - beware. If this is a typical Dutch birthday party where the whole family comes, then it is probably not what you think.
The whole family is 'gezellig' sitting in a circle and anyone can talk one by one. There is of course cake but don't assume that it will be a wild afternoon. So ask what you can expect on the birthday before you accept the invitation!