Well, to be honest, I can kind of see their point. A one year old will not enjoy DisneyWorld. We live an hour from there and we've decided to wait to spend the money on visiting a park when DS turns 3.
It costs about $5,000 to spend 2 weeks at Disney and I can see why they'd try to encourage you to wait until your son can really enjoy it and have memories of it. Of course it's your money and you have final say.
But the 6yo will enjoy it. Should she have to stay home because Baby won't get as much out of it?
If LilacGirl wants to use her money to build a caravan of neon green Popemobiles, it's up to her and her DH. Grandparents don't get a say in how an adult couple spends the money that they earn. In OP's shoes, I would simply stop telling GPIL things. Don't mention that Baby will be one. Don't bring up Disney again. If GFIL tries to bring it up again, I recommend the copy and paste (I know there's an ehell phrase, but I don't remember it.) method;
GFIL: Baby won't even remember it
LG: So you've said, but our vacation plans are not up for discussion
GFIL: You need to save for a house
LG: So you've said, but our vacation plans are not up for discussion
GFIL: It's a waste of money
LG: So you've said, but our vacation plans are not up for discussion
GFIL: If you go to Disney, the mouse will kidnap the baby and force him into slave labor on the Small World ride
LG: So you've said, but our vacation plans are not up for discussion
though I'm sure there's a better phrase. That's just what I could come up with off the top of my head.
Get DH in agreement and stop the info train. What GFIL doesn't know, can't annoy you
