It is unlikely that a request to bring another child, especially a younger one, to a school event will be declined. However, it is still good to write a message in advance so that the teachers can make preparations. Adding how you and your child are looking forward to an activity gives the impression of your cooperative attitude towards school activities.
Conversational flow and useful expressions
Your thoughts and feelings
Whether it is all right or not
Attaching "~chan" to a girl's name or "~kun" to a boy's name is usually only done when addressing or referring to children besides your own. It is not very common to attach "~chan" or "~kun" to your own child's name, especially in public.