Tell HER to head for the Shoreline...that's where the sand's at!
SLD:
Both you and your ex have a financial responsiblity to meet the NEEDS of your children. These basic needs include housing, clothing, food, transportation, utilities, etc. Toothpaste, TP, and hair gel are also assumed to be part of the "lifestyle" that your children may have been used to and is also part of what would be covered by your share of the base child support.
The amount of support you pay may be influenced by the amount of time the children spend with you. If you have the children for approximately half of the month, your support obligation to the ex may be less, as the amount of your direct payment (for food, transportation, etc.) is assumed to be greater. However, it is cutomary for the primary residential parent to be responsible for basic purchases (clothing for example.)
I think the wording of your agreement is pretty straightforward. These are EXTRA-curricular activities, right?! Sure, it would be nice if kids could have everything extra. If you BOTH can afford it. I can afford it -- but my kids know that Daddy-o is tight. So, we play board games instead of going to the Suns game -- and that's life.
So, if you can't afford it -- don't. And don't feel guilty if you can't -- that's life, too. Like you said, you have to make ends meet. So, maybe while things are financially difficult, you and your ex could manage to agree on a single extra-curricular activity. I recommend, given her "you will pay" attitude, that you consider communicating about this in writing. CYA. Let the ex know if you have a DESIRE to have the children participate in XYZ activities but also communicate what you can/cannot AFFORD at the current time.
Although most would like themselves/children to have the same "lifestyle" they MIGHT have had if divorce had not occured, the overwhelming majority of divorced families cannot. Give yourself a little time to get back on your feet and you may be in a better position to help your kids to be involved in more activities. Until then, let the ex TRY to MAKE you pay. Then tell your story to the judge...you'll be OK. Good luck.