You get that there. It also tells you in the website. (Formulario F-001 (Gratuito). Es generado por el Sistema de Migraciones en la ventanilla de atención.)
Just the payment, original and copy of the DNI, both of the kid as of the parent. ( I think they only ask for the DNI of the parent that accompanies the child )
You can go to Ovalo Gutierrez. And with children you get the fast lane.