I would report the cyclist. Child endangerment. From the description there seems to be no true reason why the kid should be on the cycle? There are laws to protect children from acts of stupidity. If there is clear and present danger related to the child's safety the law can step in and through legal processes do something to abate dangerous behavior on the part of the parent.
That said - it will take time because it becomes a legal matter.
Sometimes you wonder how children make it to their teen years when you have a parent(s) unable to make simple choices related to their health & safety.