No, the previous parents cannot get a child back after an adoption is complete. Adoption is a legal process by which the parent-child relationship is permanently formed. Before a child is eligible to be adopted, the parental rights of the former parent must be terminated, or the former parent must be deceased.
The adoptive parent takes on all of the rights and duties of being the parent. So, once the adoption is complete, the adoptive parent can even have his/her name added to the child’s birth certificate. This cannot be undone, and the former parents can not get the child back.