When you're calling up potential birthday entertainers, treat them like employee interviews. Don't hesitate to inquire about experience, references and what's involved in their usual act. Above all, ask about their comfort level around children and whether they specialize in kids' parties. For example, a magician who normally performs at corporate events may not be the best fit for an elementary-aged group.
Also consider the length of time the entertainer plans to perform. Younger children won't be able to sit through much more than half of an hour. Preteens may do better with hands-on activities and plenty of audience participation.
Before you're ready to dial up an entertainment service, do your homework. Have the date, time and location already decided on. Be able to describe what type of entertainment you're looking for and whether it should fit into a theme. The number of guests will also be helpful in case rates fluctuate depending on the head count.