I am sorry to hear that you are having a tough time and need to cancel your sponsorship. I will be praying for you and your child. To answer your question, I am thinking you could contact Compassion once you log into your account. Just click on the Contact button at the top. It says you can Chat to someone "Live".
Here is info I found on here : "We are here to help! If you need to stop sponsoring, please post here on this online community, email us at firstname.lastname@example.org, or call 800-336-7676. We will work with each of you to see how we can best assist you in your situation.".