i am the non-custodial parent and my ex has been w/holding my kids unless i do a drug and alcohol test each time before i drive my kids.

I have never been convicted of any alcohol or drug related charge, but my ex is will not let me take my kids unless i purchase and take a drug test. I did make a poor choice and poured a beer in a plastic cup and was going to drive my 16 yr old to get school supplies. I dumped the cup out, but my daughter told her mom and now she says i could be charged with a felony. My question is am i allowed to take my custody agreement and the sheriff and go get my children for my visits. My family says i shouldn't take a drug test to appease her because it only reinforces her control over the situation. How can I get my ex to honor our custody agreement without going back to court?