It would be in the best interest of your children that you go to court and deal with this. Please bring any and all documentation you may have including text messages, emails, and receipts.
Prepare to get yourself an attorney for the future. This would also be in your best interest. Your ex will need to have proof of the child support he has paid in the form of cancelled checks or cashed money orders.
I would also advise you to request the court take the child support directly from him through his pay source. This would be better tracking of what he is or isn't paying.
Also understand he may also request a change in the visitation order or custody order to also attempt to get out of paying the percentage he is required to pay. Just be prepared.
RECAP - Go to court. Bring your documentation of events (saved emails, text messages, etc), and request the court withhold your child support
Lastly, court is no place for you to be emotional. The judge doesn't want to see you being overly emotional even though this is a passionate issue. Stick to the facts, he was supposed to pay x and you haven't received x.
You should do everything you can do to get yourself an attorney because in my experience judges treat people in your position with attorneys better than those without legal representation. Make that sacrifice because it will benefit your children.