There's a first time for everything, and your son's Bar Mitzvah is too precious an occasion for it to be the one day disaster strikes. If your DJ is sick or has an accident, it is unacceptable for him to apologize a week before saying he can not attend and offer a refund - he should provide you with a DJ of equal caliber. Most larger companies will have backup DJs that can fill in during a mishap, which is very important to know. Furthermore, make sure that your DJ carries a cell phone or pager so that you can contact him directly if there is a problem or a last minute change of plans. Calling their business line isn't very useful on your big day if he or she is stuck in a traffic jam.
Most companies do a lot more than just play music.
They can do anything from vast multi-colored intelligent light shows, to include a parade of dancers, pass out party favors, or project videos on the walls. Many DJs have a list of games and activities for children such as Coke and Pepsi and Huggie Bear - choose what you like. Everybody needs to seek the restroom sometime or get a glass of water - this does not mean there should be dead time during the party. There should still be music playing, and dancers or the MC should still work the crowd. Consider whether you would like your DJ to supply music for the cocktail hour, or maybe you prefer someone more classy such as a pianist or violinist.