Your ex-spouse has always been good with your kids. You have both been working together to raise them, and all has gone pretty well, considering the situation. You buy them items outside the child support you already supply. You're thankful to be in a position where you can do so.
What you're starting to worry about, though, is that the support you want to go to your children doesn't seem to be enough. Your ex keeps asking for more money, and she said she'll have to seek a modification if you won't pay. Nothing about her situation, or yours, has changed (that you're aware of). What should you do?
All in all, knowing that you pay child support on time and pay what is asked of you is a great point in your favor. Additionally, because you spend even more on your children outside what you owe in support, you can be seen as a real team player. You don't just do what your responsibility is legally, you go above and beyond. That looks great to a court.
In terms of getting more, your ex-spouse might try but be out of luck. Unless there is some significant change in either of your circumstances, a judge may be wary about changing the support order. After all, this amount has been paid, and you already pay more. Make sure you bring receipts, documentation of paying on time and anything else you can bring to show that your financial situation has not changed (and that you don't think hers has, either). With this kind of information, a judge can make a well-rounded decision on if an increase is really the right choice.