Many couples honeymoon there. A man I work with and his wife go 5-6 times a year and they're middle-aged.
DH and I spent part of our honeymoon there.
I think everyone is so focused on the 1 year old that they've forgotten that there's also a 6 year old, and 2 adults, involved. So what if the 1 year old doesn't get much out of it this time round? The 6 year old (and the adults) will, not to mention the 6 year old and the adults will be making memories.
Living in New Zealand as a child, Disneyland was The Ultimate Place In The Whole Wide World To Visit. I would have given my right arm to visit as a 6 year old/any age child. We nearly did when I was 9. I may one day forgive my parents for taking us to Australia instead. Or not.
My first visit was as a 20 year old. It was ok; I actually enjoyed it more on later visits, and am quite addicted now. It also made me very determined to take my (then-future) children there when they were kids, which I'm pleased to say I did (both Disneyland and Disney World).
If OP can afford it, and isn't asking anyone else to foot the bill for this trip or any other living expenses, then it's no one else's business, least of all a rude FIL's, what they do with their money. (BTW, OP, I can highly recommend the Pop Century as an onsite hotel).