Can I get more child support if I have a high-needs child?

You should not be using only a meager online calculator to fix the value you place on your children. If the child is under 18 (or 21 attending school), you can fill out a rebuttal worksheet to show that increased budget your high-needs child requires. If the child is over the normal child support age or you just want to plan for that time, you can negotiate a higher amount, put it in writing, and it will bind both parents.