In reply to alj:
Mills hasn't been to a crag yet, but I can't wait to get climbing again. If you stay for longer at one point, it might make sense to put up a travel cot as a play pen. We've got the Little Life Arc2 which weighs only 2kg or so, works like a tent (including pegs for outdoor use), and packs really small. You can even buy an extra UV cover for it.
Its bigger brother, the Arc3 is 4ft by 4 ft (and psd made me return it on the basis that it was too big to be used anywhere where you'd need a travel cot. I hate it when he's right) and probably would be a brilliant outdoors baby cage.
I'm by the way not suggesting that caging babies is the way to go, but it'd give them a place to kip which is relatively sheltered and make it easier to contain them.
But then I'm not talking from experience yet, as all outdoors experience we've been having so far is walking to Cwm Idwal, and such like things.