I have 3 children myself….two of which already come with me on trips (3 year old hasn't yet).
I also agree that generally I just bring adult bags. Mainly…b/c:
1. That is generally much more available on the swap.
2. Only a small weight penalty.
3. Gives me more options when not taking the kids (I can get a greater selection of bags to choose from for myself).
4. Gives a good gear storage area…just place packs on the foot of the bag to decrease length.
Though….I wouldn't hesitate to buy a childs bag on the swap….just means I can resell it back on the swap for a marginal loss (or profit if you get a good enough deal!) when they outgrow it.
However…..I wanted to give you another option. I just purchased an EE accomplice, which is a well constructed piece of gear. I can't give a long term review, b/c I have only used it one night with my 7 year old.
There is another active thread about them here ->
http://www.backpackinglight.com/cgi-bin/backpackinglight/forums/thread_display.html?forum_thread_id=91622
So…accomplice. Saves a lot of weight, bulk, and …cost. Especially, since right now they are cheaper than they should be. Buy one now…and you'll get instant equity b/c the price will be $30-$50 more starting in November.
With an accomplice, you just carry the bag in your pack. Your child can then carry the tarp, or inner net, or sleeping pads, etc.
It also gives me the ability to control the child during the night without getting out of my bag. My 7 year old moves a ton during the night (unlike my 5 year old), and now I can just reach over and pull him back up or flip him around (yep…he'll be reverse eventually during the night), without unzipping,unbuckling my own bag.