Wow, a lot of People Like Me here.

We are six, three sons, youngest is my only girl, age 2.
You mentioned that the youngest is 2, but not the others' ages. We made trips to DLR and are going to Universal for two days this December before we start our 10-day WDW stay despite people talking about how we should wait until all the kids are potty-trained, until they're all old enough to remember, etc. I reject that. My elder kids deserve to go when they are kids, while things are still really magical. So we did DLR as a family of five to see Radiator Springs when my eldest was about to turn 8 and still loved McQueen and Mater. Now we're going to WWHP as a family of six while he still thinks getting his letter from Hogwarts would be pretty cool.
So I totally understand why you're doing a day at Universal, despite the cost. The kids will lose their minds at
MNSSHP with the trick or treating. It's really special when lots of people come in costumes and your ToTing with a castle in the background.
As for budget, I use the Dining Plan as my guide. We are going during a
Free Dining period, but if we bought the plan, it would be about $240/day plus a ~$40 tip for our Table Service meal of the day. So, I think making room for $300/day gives wiggle room and helps with the ears and other merch you'll discover. I am giving each kid a $50 gift card, and we already know that we're spending that much on wands at Ollivander's.
Yes, of course, you can do the trip on less than that, but I like a generous number to give some padding, avoid unexpected debt when I return home, and allow the trip to feel like a nice vacation rather than a constant budget struggle trying to keep it on a shoestring and feel like I'm saying No 10,000 times each day.