Instead of:
The rules of the Centre are included on the registration form you completed on the initial day your child attended the Centre.
Use:
We listed the rules of the Centre on the registration form. We gave you a copy on the first day you were here.
A good test to see if your sentence is too long is to read it aloud – if you need to take a breath before you reach the period it could be too long.
This way of writing is not simplistic like “Dick and Jane” but simply using common words.