1st Peter 3 :1 In the same way, you wives, be in subjection to your husbands,+ so that if any are not obedient to the word, they may be won without a word through the conduct of their wives,+ 2 because of having been eyewitnesses of your chaste conduct+ together with deep respect.
I'm not big on the whole JW "headship" thing but as a Jehovah's Witness she is going from door to door claiming to believe that the bible is the word of god and is claiming to follow his laws and is recommending her way of life to others.
How can she make that claim if she is disregarding what the bible says and what her religion is continually admonishing women to do? What would the householder say if they knew that the woman standing at their doorstep had 3 small children and had a husband who for good reason, specifically asked her not to spend so much time in this activity until the children were older? What would they think of a woman who thinks she has her life together enough so as to be able to preach to others, yet continually disrespects her husbands wishes as he tries to lead his family down the road of life?
I would sit her down and read the scripture above to her from her own Bible and ask her the questions I've posed above and tell her once more what your wishes are.
I have a feeling she may need a reason to get out of the house and to be around other people and Pioneering is a way to do that and get accolades from her JW peers at the same time. You don't want to make her feel trapped and enslaved by her marriage to you and her duties to the children but someone has to take care of them when they are this small. I can't see how she could pioneer and be driving around in a car all day with small children, nor do I think it would be good to leave them in the care of someone else and have them do her job, just so she can get out of the house.
Find out if being bored or stuck at home is at the root of this problem. Maybe you could arrange things so that one day a week she is freed up do whatever she wants, whether it's going in the door to door work or going out to lunch with a friend.
Let her know that this isn't a power play on your part or about you wanting your way or wanting to boss her around. It's more about the two of you wanting to come to an agreement about how to manage your family and about her following through with what you've both agreed on.