Welcome to Android Central! I assume this is Facebook Messenger Kids, right? You're saying your child sent a request to someone else -- so that someone else is telling you they can't open the link? How are you seeing a link if you're presumably not the one who's receiving the request?
I am having a similar problem with a link sent by my sister for my nieces. I seem to have connected to them but am never able to open what she has sent me. It simply says "can't open link".