WTF? What point does this prove? Lets ignore why they were in contempt of court for a second. That sentencing, even for an an adult, is beyond normal sentencing for contempt of court. For the 11 year old, that would be 7 years in prison. Typically, the period ends once the individual does what the court asks.
Now, getting to the fact that these are minors, I don't know how the legal system allows this to happen. They aren't adults. And for the most part, this is a civil matter, not a criminal matter. Most children wouldn't have the first concept of what being in contempt of court is.
I also don't get what the judge is trying to accomplish. How does this get the kids to agree to spend time with their father? Wouldn't a more pragmatic approach is to order the children to counseling to figure out why they don't want to spend time with their dad? Maybe start off with supervised visits first?