You may write a note informing the school that your child is feeling better and may take part in swimming class. You may also write a note such as this if you would like your child to resume swimming after having been excused from it for some time. It is not necessary to give specific examples of what your child says, but doing so will give the teachers a better idea of how your child is doing.
Conversational flow and useful expressions
Whether you wish your child to take part in school activities
Child's words/behavior (optional)
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.