You do not have to be sure. You do not have to explain why. Reporting something that turns out to be nothing is always the right call, and nobody gets in trouble for it.
Call 999. Do not wait for us. If you are worried about a child but it is not an emergency, the NSPCC helpline is 0808 800 5000 and Childline, for children themselves, is 0800 1111.
Email safeguarding@cookie.education. It reaches our Designated Safeguarding Lead directly, and you do not need an account.
If you do have an account, signing in lets you report from inside the product instead, but please do not let that stop you. An email is just as good.
If you would rather write to a person, email safeguarding@cookie.education for anything about a child's safety, or support@cookie.education for everything else. What we do about safeguarding explains the rest, including a route that does not come back to us.