My daughter is 14 years old and I have shared custody of her. She lives with me 3 days a week. My ex wife claims my daughter as a dependent. My daughter is covered by my extended dental and medical benefits from my employer. Can I claim the uninsured portion (i.e. the portion I paid) for her dental braces?
Yes you can claim these expenses, provided no-one else has, and that the expenses are eligible. You need receipts for amounts claimed, which would be any un-reimbursed costs.
You may be advised to claim for your daughter as a separate dependant, with credit on line 331 not line 330 of your schedule 1. This men's that 3% of her Net Income will be deducted, not 3% of your Net Income.
The downside to that approach is that you have to claim your own Medical credit (if any) separately on line 330 of your schedule 1.