<3 yrs: I recommend child in mothers arm, immobilised... while one injects in thigh or butts.
3yr and above : no amount of counselling helps. Just inform that he may be given an injection which may not pain if he cooperates. Then apply restrain and finish the procedure in shortest time without the kid realising most of it.
The maximum pain occurs when the chemical brushes with cut nerve endings.
use a separate needle to inject.
and when you withdraw needle out, let it be in shortest time span ( preferably with a jerk... rather than slow removal that pains alot).
injecting medicine in... slow or fast usually doesnt much affect duration or intensity of pain...( u r in painless intermuscular or fascial plain)
avoiding bony sites and periosteal injury will be preferable. ( so prefer bulk of muscle... dont hurry to inject)