In cases with extensive decay, we are limited by the maximum dosage of local anesthetic that we can use. As a rule we also consider your child's comfort after he/she leaves the clinic, in order to determine how much local anesthetic we can use.
Very young children are at high risk of biting their lips or chewing on the inside part of their cheeks after they receive local anesthetic (a lidocaine shot). This usually happens because of their natural curiosity they try to feel the area or areas that are numb.
For these and other reasons it is unlikely that we could work on all of your child's teeth at once. An exception to this rule would be a child that is taken to the operating room.