Assuming the querist means that she wishes to take custody of her child without using proper legal procedure, it would most assuredly not be advisable to do so.
The term for a non custodial parent taking a child without permisison of that child's custodial parent or legal guardian is 'parental abduction'. Regardless of the nomenclalture, it is a criminal offense and the perpetrator could face numerous and serious charges.
If there is a question about custodial rights a parent should discuss the issue with a qualified attorney and should never "take matters into their own hands". The obvious exception would be if the child is in a situation of neglect and/or abuse and/or endangerment, then the parent or interested party should seek assistance from the jurisdictional authorities.