To put double glazing into aluminia frames is a waste of money . Aluminia is a bad insulator , and plastic window frames cost very little. If you do just the two in the kids room that won't cost the world. Try to get the best available in terms of insulation properties , you might save enough on fuel and renovation costs and antibiotics ( condensation ! ) to finance in one or two years the next set . Talk to your window maker , may be they accept payment by installments . Good luck .