We've always just gone for putting a space blanket between the inner and outer tent skins. Keeps it warmer at night, cooler in day, and much much darker in the morning, and only costs a few quid.
That was when we used to go in a 4 man one room tent with 1 DC though so not sure how well it'd work with a bigger tent but I guess a bit of gaffa tape <my answer to most things> and a couple of space blankets would do the job.
Is there such a thing? DH says he has seen some, with animals on the canvas and made of blackout material, but I can't find them anywhere. We have a Vango Tigris 600, and are in desperate need of camping sleep with a 2 and 4 year old..