Unfortunately, it does seem weighted in the mothers favour, and I'm afraid it will be an uphill struggle, but the mandatory reconsideration is the right direction. I don't think showing the bank statements of you paying for uniforms etc will make any difference (though won't do any harm to do so) as they will just say that it's up to you to pay those extras on top of maintenance.
If you can each claim child benefit for one child, that would help immensely (also, you'd both get the first child rate, which is more than one person claiming for two children - perfectly legal to do so), and you'd then open a claim against her so you'd be paying maintenance to her based on your income, and she'd be paying maintenance to you based on hers, both of you for one child. CMS are concerned with the number of nights that the children stay with you (doctors appointments etc are irrelevant as far as they are concerned).
If they do continue to expect you to pay maintenance, then stop paying for anything else - the maintenance covers everything, and anything you pay for will be considered to be voluntarily on your part over and above maintenance.