Quite simply tell them it's not them that's having to wear the baby or help him to sleep so they don't need to worry about it. It's worked for me. Then it's a matter of patience... proof is in the pudding.. when they see your well adjusted, well sleeping baby/toddler, they can't say anything anymore.

People know not to comment on DD still waking through the night, because *shock horror* it wasn't a lifelong bad habit after all with my DS! LOL. Just be confident, thank them for their concern and say you're happy doing things this way