When you say you have manually paired them, have you generated a pairing code for the parent? It might be easier to advise knowing what you did to pair them.
As for pairing codes, there are several ways to do this: https://community.canvaslms.com/t5/Canvas-Resource-Documents/Pairing-Codes-FAQ/ta-p/388738 Some of them require your institution to have given particular users the ability to generate a code.
It is possible for them to get a link through their student's account. They just have the student sign in and they can generate a pairing code by following these instructions: https://community.canvaslms.com/t5/Student-Guide/How-do-I-generate-a-pairing-code-for-an-observer-as...
