Undies aren't the problem in my opinion, and why the hell should the innocent be hidden just because there's a lot of weird pervs in the world?
Saying that, I say no to posting it on FB, purely because you lose all control... and control is something you should want to keep for as long as possible. If you got a picture developed in Asda and they then used that picture in the advertising campaign without permission, you'd be fuming right? Once you post your pics on the www, especially FB you lose ownership.
I do not buy into the paedomergencies AT ALL, and they make me rather cross in all honesty, because people who spout 'what if a pervert has a wank over it, you'll be to blame' are basically sayin that the parents of the children pictured on nappy wrappers are bad parents, and are 'asking for it'.
We should protect our children of course, but we should also protect their right to BE children and prance about in their undies freely, and celebrate it. Anything else actually allows the sick few to ruin the beauty of childhood for everyone else.
op You're not a bad parent at all. I have a similar video of my now 5 year old totally starkers, and it's brilliant, natural and funny as hell. I'd happily show you it, I'd happily show everyone, but I wouldn't post it on FB, purely from an ownership PoV.