So Your Kid Wants to Be a Pagan, Too.
Teaching children spellwork can be a very dicey proposition. As a Grandfather and a member of the BTW (Gardnerian) Priesthood, our policy is to not teach anyone under the age of 18. older if they are not mature enough or ready to train. That said, it is no different than teaching your children about anything that has the potential to do great harm; or great good. Start with the basics of discipline and conscientious use of power. Without discipline and conscientious use of power all other training is for naught.
This is difficult for self-taught eclectics with Pagan children as they are doing it all without the benefit of an experiences HP/HPS to guide them and give them any potentially necessary pointers; even more so when the parents themselves, are self-taught and lack the discipline that one gets by working with an initiated and properly trained teacher.
In a real sense, it can often be the blind leading the blind. However, children can be very adept and powerful magic users, when guided properly an grow to be very effective and responsible magic users as adults, when guided and trained properly.
My wife's Goddess son is one of those children. He is allowed to participate in the rituals for our major Sabbats and that is the limit of his involvement with the craft as far as what we do on a regular basis.
Outside of that, his parents, who are outer court students, themselves, have asked for guidance from us when they run into the tough questions and sometimes he will come to myself or my wife/HPS when he has questions about the use of magic and the moral consequences for abuse of that power.
The whole spirit guide thing is interesting, but a bit overrated. Few witches have a spirit guide. That is more for shamans and is better served when following 1st nations shamanism and animism rather than those traditions being butchered by well-meaning non-tribal members (usually us white Euro-American folks).
Spirit guides are also helpful for those doing seances, but not so much for the practical use of magic as indo-European peoples practice it. That sort of thing requires levels of training that is virtually impossible for non-1st nations people. Teaching a child to channel his power, using the appropriate God or Goddess for the purpose is a good Idea, since the Gods are not jealous and some tend to specialize, thus requiring different rituals, practices and offerings. For a younger child, I would focus on simple spells like charging a good luck talisman, or simple rituals such as room/house cleansing, for example. Those are harmless things and cannot affect the free will of others.
Include the child in the rituals for Sabbats. We already do that with the Easter egg hunt every year, and in some areas, the Maypole dance, as well as (to a small degree) All's Hallowed's Eve/Samhain and even most of our traditions celebrated during the Yuletide, what is called Christmas by the non-Pagans/mundanes are Pagan in origin.
So when you do these rituals, make him/her a part of them, have him/her study the Sabbat and, if he/she is old enough to actively participate, have him/her help perform and/or be involved in writing the ritual (that is where having the child study the Sabbat comes in). That educates the child as to WHY the ritual is performed and why it is performed for that certain event. It will also help develop discipline and focus as well.
And don't be afraid include some humor, whenever practical to do so. If one is banishing bad things, the joy of laughter, or a smile, is also very powerful magic and it will show him/her that magic is a wonderful and joyous art; not just stern and serious. Let Loki/coyote poke his head in from time to time, where it is appropriate. It's not only good for teaching, but it also makes the craft work that you and your children do more enjoyable.