Step parents

What do you do when residential parent is constantly saying the child is always taken care of first and has the best of everything in their home but you know for fact and have proof that it isn't true. My step child comes over in clothes that are way too small shoes that dont fit. Step child complains and has said that mom said she doesn't have money for new shoes but gets well over $400 in child support even though my husband has joint custody. The mom is out getting her hair done nails done and just got eyelash extensions but told step child she has no money for shoes that cost about $20-30. We have resorted to not letting any of our clothes go over to moms house because we never get them back in turn causing us to spend money about every 2 months to replace the wardrobe so we stopped that. We have also tried having child support modified since the amount is the same since they split which was 7 years ago and at the time the mom had no job and my husband didnt have joint custody but the agency is saying my husband can't request a modification without the mom okaying it because the file is opened in her name. She won't have it modified because she makes more money than him and doesnt want to pay child support to him even though we dont want it we would be happy if it was just lowered to fit the current order and would be ecstatic if it was just dropped as we would finally not struggle paycheck to paycheck. We just don't know what to do to go about getting help to change it so we can support the child in our home the way his child deserves without us going into debt even more.