If a child is abducted to a Hague Convention country an immediate application must be made to the International Child Abduction Central Unit. There is a standard form that can be used for this which can be obtained on the link.
As an initial point legal aid from a solicitor in the UK will not be available and your case will be dealt with and handled by the International Child Abduction Central Unit, who will make contact with the International Child Abduction Unit in the country to which the child has been taken. A solicitor will be appointed for you in the country to which your child has been taken and an immediate application under the Hague Convention will be made for the return of the child to the UK.
It is sometimes relevant to apply for other Orders at the same time in the English Courts and expert advice from a solicitor should always be obtained.