Our first trip was with a 6 year old and 2 year old so I feel uniquely qualified to respond
- We chose to stay onsite at Pop Century. For us it was a great choice because while it cost a bit more, we didn't have to worry about bringing car seats on the plane.
- You don't need a full kitchen. We did an instacart order with common breakfast items and snacks and had breakfast in our room every day. That plus mickey bars plus dinners in the park/hotel were enough!
- We booked a car that first trip for one night only so DH could take DD to Blue man group at Universal. It was honestly more hassle than it was worth. Unless you're planning to stay offsite or leave the bubble, you don't need a car. See previous comment about carseat!
- Groceries are easy peasy - any number of delivery services available. We used Instacart/Publix but there's lots of options if you search the boards.
And not that you asked, but I'm going to give some unsolicited, highly controversial advice (don't believe me, check the family forum) - get a double stroller. Your 6 year old might be the most independent active little soul at home, but trust me when I say that it's not worth the fight over the stroller with the 2 year old who definitely will need one. Our 6 year old napped in it during fireworks alongside her brother. It was worth its weight in gold that first trip to us!
One more piece of advice - seriously, don't try to do it all. We went for a week and had 5 park days - it was too much. We had very over stimulated cranky kids. Don't worry about getting on every ride. Pick your top 5 and everything else is gravy. And have so much fun!! Age 2 was magical because the character is very much "real" to them!