“There is no limit, in legal theory, to the jurisdiction of the court in England to act in the interests of any child who happens to be within the jurisdiction for whatever purpose and for however short a time. In practice, however, if the child is not habitually resident in this country and there are legal procedures in the country of habitual residence available to achieve a fair hearing of competing parental claims regarding the child's upbringing, the English court will decline jurisdiction, except for the purpose of making whatever orders are necessary to ensure a speedy and peaceful return of the child to the country of habitual residence. The practice thus is to follow the spirit of the Convention, even though its formal terms are inapplicable.”