If you have kids and you're growing, money should be the least of your concerns when buying items for security. Spend the money upfront and protect you and your family, choose not to grow, or grow and risk being caught and your family broken apart.
There is a very reasonably priced prodcut called Poly Shield. It is a thick poly that is black and white with infused metals that are highly reflective and insulative to shield against infrared detectors.
Each roll is 6' x 100'. The price per roll is $180 + shipping. A very reasonable price to pay for piece of mind and security against big brother.
You will need enough to line your entire loft, or I suggest building a smaller section inside the loft and using the poly shield as the walls. A simple wood frame construction with poly shield wall panels stapled up would be great. You will also need to seal off all the cracks though and overlap ends to ensure everything is insulated.
The great thing is that you use the white side facing inwards so you don't even need to add reflective mylar for your lights. The walls will already be white. You could if you wanted though and I'm sure that would only add to the insulative effects of the poly shield.
Bottom line...You have a family to think of here. DON'T SKIMP OUT ON QUALITY SECURITY!! Save up your money until you can afford it and then buy it. It might take more time, but the peace of mind is absolutely worth it. Especially in today's world of blatant privacy intrusion.
George Orwell sure hit the mark with 1984.