Deductions & credits

My question is this:
Is there a cap or limit to how much I can claim on credits? Because I had been playing with the software trying to understand why am I not getting the credit and I observed that if I delete the car, my tax due doesn't change much because my child credit goes up, but if I include the car on the tax then I get 25% of the $4,502 (Personal portion, 75% business use, 25% personal use) but at the same time my child credit goes down by a similar amount, for what my tax due only is affected by $43.00. I thought that my tax due was going to go down for the entire $4,502 and as I paid estimated payments I could get some money back. Am I totally wrong with this? or can it be a glitch on the software perhaps?