I don't actually do invoices as the amount is the same week to week. Parents just give me a cheque and I write a receipt using a basic receipt book that has duplicates so I have a record too.

If you look in Word or other processing program you may find a template you like.

Parents' name
Child's name
Dates of care
Amount owing
A place to mark (your signature) when it is paid
Mark method of payment parent used
Have some sort of number on the invoice so you can file them easily (having the date at the top will work or number by month week such as August - week 1, August - week 2

That is basically all you need on the invoice.

You can add your address if you want
You will still need to issue an annual statement with SIN or BN number unless you put that on each invoice.