i haven't read all the responses so i am sorry if i am repeating something someone else has said, but could you just nip it in the bud by addressing it as soon as they say something about your parenting/kids/whatever.... you don't have to say it in a mean way, but more just along the lines of 'can we not make comment on the differences in how we parent our kids, obviously i do things that you wouldn't, and vice versa, but the beauty of it is we all get to do things that work best for our families, so lets leave it at that'
hmmmm that didn't come out right, but you get my drift.... also i wouldn't be able to resist saying some things that they do that YOU don't agree with, but only in response to them commenting on your parenting first though, just to illustrate that everyone does things differently.... if that makes sense... they might stop so freely giving their opinion if they also get your opinon back in response, all said with a confident smiling face.... they are clearly trying to make themselves feel better/superior by belittling the way you do things... obviously insecure or something. or pathetic.

good luck let us know how you go.