a somewhat contrary view...
personally, i'd not bother - i've done the family camping in winter thing, Rothiemurchus forest, near Aviemore in February (if you're interested), with two young kids and all the tasty gear: Rab 4 season down bags, proper winter clothing, a mountaineering basecamp tent, thermarests, fleecy suits and waterproofs for the kids - and the grim truth is that just being there is too cold for the kids. the sitting around waiting for dinner to cook, the getting dressed/undressed, the anytime-you're-not-actually-doing-anything time is actually really, really cold for kids. they loved the snow, the views of the mountains, the being in the forest, but it was just too cold.
we camp lots in spring, summer and autumn, but in winter we go for the camping pods and the like - its just a lot easier. or, of course, a caravan...