The obvious answer is to present a case in the American court to allow that proposed arrangement.
Taking the child out of the US court's jurisdiction and not returning it in due time would be seen as a) a breach of a court order, punishable on the offender's return to the US b) possibly kidnapping under US law by taking the child outside the jurisdiction without consent of the other parent; this is an offence here, created to meet cases of parents who snatch the child and keep it abroad [Child Abduction Act 1984 section 1] but the Act applies only to taking a child from the UK.