As you mentioned volcano, the child is two and is hopefully being sat in a suitable restraint system, I assume like this
one. If this is the case then there is no real requirement to disable the airbag and my advice would be not to!
The guidlines state that it would be prudent, although not necessary, to seat a child in this type of restraint system in the rear of a car especially if an airbag is in use, however, I believe it would be safer to leave the airbag active rather than inactive.
Put it this way. If you, god forbid, have an accident would you rather some airbag injuries to your child or possible worse injuries? I know which I would prefer.
for further guidlines on child seats