The Secretary of Health and Human Services shall take all possible steps to encourage and assist the various States to enter into interstate compacts (which are hereby approved by the Congress) under which the interests of any adopted child with respect to whom an adoption assistance agreement has been entered into by a State under will be adequately protected, on a reasonable and equitable basis which is approved by the Secretary, if and when the child and his or her adoptive parent (or parents) move to another State.section 673 of this title