Where I practice, the two of you are likely operating in the gray area in between separation and divorce when there is no order in place regarding child custody, visitation, and child support. You haven't been ordered to pay child support, so legally you’re not obligated to pay it right now.
With that said, if the issue was brought to the court, you would likely be ordered to pay it, plus arrearages, so it's a good idea to keep paying so you don't fall too far behind.
When your child support obligation is calculated and ordered, anything you've paid in the past will be offset against any arrearages you may owe. This means you get a credit for what you've already paid against anything the court deems you owe from the past.
How Much Should You Paying?Child Support Calculators
Now, with regard to your wife's attempts to deny you visitation with your children, you're sort of in the same boat. There is no order in place saying who gets them when, and for how long, so she's not legally in violation of any court order.
Now, you may be able to rely on your past arrangements with her to show the court a precedent has been set for visitation. She's also making a potentially serious mistake by keeping your kids from you. Judges do not like that at all, especially when the reason you're being denied time with your children is money.
It would be a good idea to get an order in place regarding child support and visitation soon so you know exactly where you stand on these issues and so that your rights and obligations are clearly defined. Once you get an order in place, you can take action any time she denies you visitation with your kids.
In the meantime, I’d strongly recommend you seek the advice of a skilled mens divorce lawyer who can help you get these orders in place, and who will be familiar enough with your state's divorce laws to help you find any immediate steps you can take to address the problem right away.