I have not tips about parking other than to take a picture of where you park! The parking lots are huge and by the end of the day after seeing so much it may be hard to remember where you parked that morning.
A lot of the answers to your questions depend on (1) - How tall your kids are and (2) - How adventurous your kids are. You can check on the websites of the various parks to see how tall your kids are. If they aren't super adventurous you can google each ride and find videos of them to see if it is something you would be interested in. If it were my kids I would do 2 Disney, 1 US, 1 IoA and 1 LL.
We took a trip to Florida this past summer. It was my kids first trip to Florida. We stayed onsite for a night at US and offsite for the rest of the week at a 2 bedroom time share. We had a 9.5 yo boy, 7 yo boy and 4.5 yo girl. My oldest and youngest are both very tall for their ages. My middle and youngest are both very adventurous. My oldest is a little less adventurous. We went to MK (2 days),
LegoLand (1 day), SeaWorld (1 day), US (1 day) and IoA (2 days). We also spent a half day at DTD and 1/2 day at the resort pools.
At MK all of my kids were tall enough to ride everything (even Space Mt). 4 yo didn't ride Space Mt, not because she didn't want to but because she wanted to meet Anna/Elsa more and I could only score 2 FP+ for Anna/Elsa, so we went there with our FP+ while everyone else had FP+ for Space Mt.
At US/IoA we got buy 2 day tickets, get 1 day free. Our tickets were not park-to-park tickets, but we aren't into HP so not going on the train between the parks didn't bother us. The non park-to-park tickets were a lot cheaper and allowed us to stay onsite at US for one night. Staying onsite enabled us to get free express pass which going in August was so worth it. We would not have been able to do 1/2 of what we did without EP in the busy summer. Our DS1 was tall enough to ride everything in both parks, but he isn't as adventurous and choose to skip the Hulk and HP coasters (can't remember the name, but there are 2 colors of them). He did ride the Rip-Ride-Rocket coaster. DS2 was too short for several of the coasters, but was able to ride the Hulk, Mummy, etc. DD was too short for all of the coasters, but could ride MIB, JP, the two water rides and a all other rides. She rode everything that she was tall enough for.
My two boys are obsessed with Legos so going there while they were still young enough to enjoy it was a must. All three of my kids were tall enough to ride everything, including the 3? roller coasters. All three of my kids did ride all of the rides there that we had time for. We were there from park opening to park closing and still didn't have time to do everything, including the Chima water ride because it kept closing down (not sure of the height requirement on that ride since we didn't ride it).
At Sea World DS1 was tall enough to ride everything. DS2 and DD were not tall enough to ride the 2 larger coasters, but were tall enough to ride the water ride and all of the smaller ride/show type rides.