I teach catechism to grade-school children. I noticed that one of my students does the sign of the cross with the left hand because the child is left-handed. I’ve tried correcting it but since the child is left-handed it’s hard for him to do it with the right. Is it bad to do it with your left hand?
It is not “bad” to use your left hand to make the sign of the cross, but doing so with your right hand is customary and it is not unduly “difficult” (even for a child). Using his left hand is automatic and comfortable, but he can be gently directed to use his right hand until doing so becomes automatic. You are not asking him to become right-handed; you are just asking him to use his right hand to make the sign of the cross. Since that is an entirely reasonable request, he should be expected to mind his teacher.