In the U.S. the appeal court will not do a full review of the case. Pure questions of law are reviewed with no deference to the lower court (de novo review). But pure questions of facts are reviewed with high deference to the lower court. There has to be a clear error in order for the appeal court not to affirm the judgement.
A determination of a child’s habitual residence is nearly entirely fact based. It wil be judged on appeal by a clear-error review standard.
You can (seperately from the appeal) ask the court for a stay of the return order, to make sure that the child is not removed from the country during the appeal.