There are no recent changes to the tax law regarding the definition of a dependent. All that changed is the amount of deduction or credit that a dependent qualifies for.
@joshramsey if you were not married to the children's biological parent, then they are unrelated to you, and you can only claim them under very limited circumstances. (For example, the children must live in your home all year, and the other parent must earn less than $12,000 and not be required to file a tax return.) There may also be legal agreements between the biological parents (the exes), or you may have been unable to prove the children lived with you all year if the children lived part of the time with the other parent. We can't help you understand your situation without a lot more details.
