If I appeal to the court that the cost of adding a child to the insurance is too high (20%) can I end up worse off

I don't understand your question. Perhaps you can find cheaper health insurance (now more widely available). You might also try to file for a downward modification if your circumstances are changed. An appeal will generally result in going back to court before whatever judge/magistrate issued the first order, so that they can reassess the case. If you are objecting to a family court order, you need to do so within the proper time limit. Good luck.