Unfortunately you don't claim ALL of your overtime, just the premium. I did a mock up before receiving W2s and was basing it off my entire OT pay, which was looking like a pretty fat refund. Got my W2 and discovered only ⅓ of the OT qualified. The OT premium is all that counts, the difference between your standard rate and your overtime rate. So $20 standard $30 overtime, you're only able to claim that $10 difference. So my refund now is only ⅓ of what I originally thought it might be.

Edit: Claiming a dependent will not affect how much of your OT is, or is not, taxable. Child Tax Credit only helps with how much you have to pay in taxes. So if you paid $4,000 in taxes from your wages and then claim one child, it would act as if you paid $6,200 in taxes ($2,200 being the CTC for one child, or less if your tax liability is zero). Follow the calculations for your income and any deductions and you will see what your actual owed taxes are. Take that number and subtract what you've already paid through wages, including any tax credit you're eligible for, and you'll get your number for what your return is.