In a standard DNA paternity test, the DNA of a child is tested against the father to determine whether there is a biological relationship. A Legal Paternity test simply means there is a chain of custody which proves that all individuals tested are who they claim to be, and the result is admissible in court.
Of course some “children” are adults.
Samples for most DNA Paternity tests are collected with a painless cheek swab. A swab is rubbed in each cheek to collect cells from which the lab extracts a DNA profile. The DNA profiles are compared and if there are enough that match, the lab can determine whether the individuals are related.
In some circumstances, DNA can be extracted from toothbrushes, cigarette butts, clothing, finger nails, blood, ear wax and bed sheets. If you are not sure, please ask.