It goes further than just child support though. For example, speed limits on interstates and such. If I remember correctly, a big one was drinking and driving laws. Up till a few years ago, montana still allowed open container in a vehicle. The state was free to do that but got less federal funding for roads. If I remember correctly.
However, if states do limit fed powers and the feds try to yank the funding, it will hurt BUT we can also vote out the people that agreed to limit funding. Maybe, just maybe if enough states do this, we can start to put the feds in their place at least a little.