I think throwing in your own opinion about whether or not the OP is making an "extravagant" plan by taking her kids to Disney is completely inappropriate. It's Disney...it can be expensive, but depending on your priorities, what you value as far as family time/money, and how the OP is choosing to budget, it doesn't mean that she's blowing money.
And in either case, this is her business. Not her FIL's and most certainly not anyone's on this board. And I happen to think that the FIL could have a point, but like I said, there are ways to make it that aren't rude:
"DIL, you've mentioned several times how you would like to save money to buy a house. Have you considered holding off on this trip for a few years and saving the money for a down payment on the house? You can have the house now, and baby may even enjoy it more down the line. Perhaps you can take your 6 year old to a local park this year so that she can still have some with you guys without you having to make this big trip to Disney." And this really only if he's talking to her about it directly to begin with. To hear the news from his son and then go talk to her after the fact is still overstepping, IMO.
and then there's
"Disney? Really? Your baby is only 1. He won't remember it or enjoy it. The plan is silly and crazy and you should be putting that money towards a house instead."
Having a point is not really the issue. The OP's financial situation is also not the issue (since she doesn't use your in-laws for support). The issue is why is it anyone's business what family decisions she makes? Telling them about them does not give them a ticket to interfere. They obviously don't have the type of relationship where this kind of advice is welcome. And to me, the crux of the issue is that it doesn't appear that the FIL approached this in a concerned way at all, but instead a critical and accusatory and even mean way. Rude.