My best advice to you is to at least have an idea of your must do rides and your maybe do rides and the layout of each park. The reason you want to do that is so you have an idea of what you might want to do next and you don't spend every five minutes looking at a map and trying to figure out where to go next.
If you don't do that, then you are in danger of not doing nearly half as much and coming back complaining that Disney is boring and busy and you don't get why people like it.
Just doing the research helps, but you dint have to do a minute by minute plan.
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And know your audience - in this case your kids!
For example, if I were taking my 3 year old niece - I would know that she is a princess fanatic, particularly Ariel and Rapunzel. She also loves Minnie Mouse. She would also have a hard time passing up the visually appealing rides.
So, a morning at MK with her might look like this: Meet Rapunzel, Meet Ariel, Ride the Little Mermaid Ride, Ride Dumb, Meet Daisy and Minnie, ride the carousel and It's a Small World. If I got there at park opening, it's probably not even 11 yet and it wouldn't matter what we do the rest of the day as we've hit our must do's.
I seldom plan out my whole day. BUT, I do plan out the first hour or two. All that really consists of is polling my family and finding out what is most important to them and doing that before the lines get long.
ETA: and as hard as it can be, I highly recommend rope drop! The earlier you are there, the less you have to worry about planning. I know it can be tough. I've got 3 kids and when they were little, I had all kinds of crazy ways to make it out of the hotel room on time. A few of them were: baths in the afternoon after swimming or at night, pack the park bag the night before, eat breakfast in the room (or at the bus stop!), have the clothes picked out, do DD's hair while waiting in line for the bus, put on our sunscreen either waiting for the bus or waiting for the park to open etc....
Since you are coming from offsite, allow time to park the car and get to the park. When I drive, I aim to be at MK at 8, DHS at 8:15 and Epcot and AK by 8:30.