For your child documents, submitting the Canadian birth certificate is usually sufficient, especially if it clearly establishes their identity and parentage. However, some immigration programs may request all available birth records, including those from the other country of citizenship.
If IRCC hasn't t asked for the second birth certificate, you likely don't t need to submit it. But if you want to be proactive, you can send it via a web form, explaining that your child holds dual citizenship and attaching the second birth certificate.