OK so the way i'm going to play this is that i'll contribute half towards large items such as school trips. I'm getting all school letters sent to me as well, so i know what half is. Probably i could pay the cheque direct to the school.
I'll also buy some clothes so they have some to wear at my house when they come over, and i don't really care if they stay at mine, i'm contributing to a pool of clothes not really bothered about the individual items. It'll be proportional to the time they spend with me.
What i am very wary of, which some people have touched on, is that she's already told me she plans to take them on 2 holidays next year, one in the UK and one away. If she gets any more money from me, i won't be able to afford to take them away myself. So the net result is that i'll be paying to have her take them, rather than take them myself, so they spend less time with their father. There are other examples like this, she's already had a huff because the Wii i'm buying one of them for Christmas is staying at mine. I don't even like the things, she does. Read into that what you will....
And to the kind thoughts of the people accusing me of being a delinquent father, probably you are vicariously projecting your own experiences onto me. I invite you to work through these important issues with your own therapist, not on an internet forum