Hi prediado,
Welcome to the Mattress Forum! 
From post #2 here, which has some good information about guidelines for children:
[i]While these are general guidelines and not “exact” recommendations because there is no single firmness level that is always best for all children and different people or different manufacturers can have different definitions of firmness … babies and infants will generally need a firm to extra firm sleeping surface while toddlers will generally need a medium firm to firm sleeping surface and older children that have transitioned to a larger mattress and are out of the crib will generally do best on a medium to medium firm sleeping surface.
Any combination of materials that are in a suitable firmness range, are durable enough (especially if you plan to use the same mattress for other children), and that are “safe enough” for your criteria, and that are in a budget range that you are comfortable with would make a suitable choice.[/i]
The configuration you’re considering would be quite firm feeling for a full-grown adult, and certainly very hard feeling for a growing child. I would certainly ask SleepEZ for their advice, but one option you may wish to consider if you’re wanting to stick with an extra-firm base layer could be an extra firm, firm and medium, and if you’re desiring a firmer surface when your child is younger you could place the firm on top and the medium in the middle, and then as your child grows and perhaps desires a bit more surface contouring you could place the medium on top and the firm in the middle. You could also go with a firm, firm, medium combination and do the same thing. This would offer you flexibility down the road.
Phoenix