Haha they are mad! I can see why they might have downgraded from travelling in Australia to 2 weeks on a package holiday in Mexico, but they're still mad.
Good luck with that Joe, I don't know who you are trying to prove anything to, you don't need to pretend your life is perfect. We all know what it's like having a tiny baby, and it must be horrific with an 18 month old on top of that (I have one 18 month old and he is hard work enough on his own!) You can just say it you know, no one will think any less of you (except us lot - we will all be drowning in schadenfreude!)