1. Westminster Shorter Catechism (NOT the Heidelberg as it is written for believers).
2. Whenever they were able to comprehend, at least in part, the teaching. (that depends upon you too

)
3. Take it slow as determined by the child(ren)'s ability to follow the lessons. Memorization is something which should be a definite part of the instruction.
There are some catechisms written for children but I prefer to use the "standard" catechisms and simple teach them in a manner which are digestible for those being taught. If you are credo, if I'm not mistaken, I think Spurgeon wrote a catechism for children which might be appealing to some.