It pays to think about this from the bottom up: most warmth is lost to the ground. Fortunately mattress technology has moved on enormously over the last few years with the advent of insulated air mattresses. These are far lighter, warmer, more comfortable and pack smaller than old style self inflatables. New models are coming onto the market almost every month, and sometimes at premium prices. However, on a tighter budget two models stand out, depending on the temperature range you need. If it is only for the summer, the new Thermarest Neoair Trekker is the budget model in their Neoair range. I weighs 570 grams, and packs incredibly small. For more warmth, I would choose the budget mat in the Exped line: the Exped Synmat Basic 7.5 at 720 grams, and a significantly higher R value of R=4.0.
If your kids are youngish, I would go for a synthetic bag as these are less fragile. We got a small size middle weight Ajungilak in a half price sale, and it has served us well, even if as with all synthetic bags insulation deteriorates quicker than with down. If they are older, I would go for a cheap down mummy. I like Alpkit for that. For you and your wife there are other options. With modern warm mattresses you really do not need additional warmth underneath from a bag. An expensive solution would be a down top bag such as I had made for my wife and myself:
http://www.tatteljee...eepersoons.html My wife's side has a much higher down fill weight than mine. It saves money and weight/volume by omitting the bottom of a two person bag. For us it was the successor to a bag system that we had used for the preceeding thirty years: a largish rectangular solo down sleeping bag with zip on two sides, zipped open as a duvet (I don't see these often from UK shops, but they are very common in Holland). Next get a rectangular sleeping bag liner for two people, and sew a matching zipper to the sides. Zip the bag to the top of the liner and you are done. Western Mountaineering (they call it their summer coupler) and some Dutch shops sell them ready made, but some homework can save a lot of money. It works fine for summer and early autumn temperatures, Beyond that, the temperature leaks from the sides will make it a bit cold. Anyway, the system saves the expense of one down bag, and the weight and volume. It is possible because modern mattresses insulate so much better. So that is why I would go for a somewhat warmer mattress.
Finally as for size/weight and where to have it. I like to keep things tightly packed, as this saves on the weight and expense of front panniers and racks. Now that our kids are older (the younger is 13 and will finally get his down bag next year), we no longer need the front panniers. It pays to consider if you can indeed avoid the expense of the front racks and panniers, and use the money saved to pay for lighter and more compact gear like down bags. On the other hand, if the kids are still young this is hardly an option. I have just come back from a week's cycling in Denmark with my 13 year old son, and it was the first time we both had full size rear panniers (before he only had small ones, and I had both front and rear panniers). I was using my 600 gram PHD solo bag, and he was using the 1600 gram Ajungilak bag he has almost outgrown for the last time. It was fun. I hope you will enjoy your trips as much.
Willem
) forever. 
