I'm not sure how should I, for making the GUI we mainly based on the ContactList example on PharoCasts: http://www.pharocasts.com/2011/02/pharo-gui-with-polymorph.html
This is my child window:

UIPrestamos>>openModal

openModal
    | builder content dialog |
    prestamo id: (Prestamo database size = 0 ifTrue: 1 ifFalse: [Prestamo database last id + 1]) asString.
    builder := UITheme builder.
    content := builder
        newLabelGroup:
            {('Prestamo: '
                ->
                    (builder
                        newLabelFor: self prestamo getLabel: #id getEnabled: nil  )).
            ('Socio '
                ->
                    (builder
                        newTextEntryFor: self prestamo
                        getText: #idSocio
                        setText: #idSocio:
                        help: 'Codigo de Socio')).
                        ('Ejemplar: '
                ->
                    (builder
                        newTextEntryFor: self prestamo
                        getText: #idEjemplar
                        setText: #idEjemplar:
                        help: 'Codigo de Ejemplar'))}.
    dialog := builder newPluggableDialogWindow: 'Nuevo Prestamo' for: content.
    builder openModal: dialog.
    dialog cancelled ifFalse: [self doOnOK].

----

Which is called this way:

UIBabel>>addPrestamo

addPrestamo
    | prestamo alerta |
    prestamo := Prestamo new.
    alerta := AlertDialogWindow new.
    UIPrestamos new
        prestamo: prestamo;
        onOK: [ ((Ejemplar  database anySatisfy: [:x | x idEjemplar = prestamo idEjemplar and: (x fechaBaja = '') ]) and: (Socio  database anySatisfy: [:y| y numeroDeSocio = prestamo idSocio])) ifTrue: [Prestamo database add: prestamo] ifFalse: [alerta alert: 'El socio o el ejemplar no se encuentra disponible'].
                seleccionado := Prestamo database size.
                self
                    changed: #listado;
                    changed: #selecciono ];
        openModal.

----

I've also found another way to show lists, using GridMorph instead of the current PluggableMultiColumnList, sadly it look really hard and lot of refactor should be done :(

> * Block a father window when a child is made, so the father window can't
> be closed if a child window exist.

childerWindow openModal: fatherWindow