We have in the past just had kids along on adult trips. Kids from ages two to teens on the same trip. We adults did what we wanted - which includes character meals for certain - and the kids just went along. We usually had a group with at least 2 adults per kid, so a too short kid had someone to hang with. Those times were often snack times. One of my absolute favorite Disney photos is of a 2 yo and a 3 yo - so colorful in Garanimal clothing - sitting on a curb in HS eating popcorn while most of the group dropped in an elevator.
Our most recent kid trip did have a 4 yo, 8 yo and 9 yo. Girls. Completely new to WDW. Their dad and grandma had been to the World a few times but not recently at all.
The biggest thing to wrap your head around is that touring will take longer with twice the people. Maybe not twice as long but it might seem like it some days. 
Another thing to keep in mind is how over-stimulating the World can be. Just walking down Main Street. The more time you can build in for just taking things in, the better the memories. Maybe. And speaking of walking: hot spot/blister checks of kid feet might be good, occasionally. Your trip is also at a potentially hot 
time of year; providing for more cool drinks and cold treats is a good idea. It’s there if needed.
A third thing to keep in mind with kids - especially if memories are the goal, and you have preschool kids - repetition. Repetition makes for familiarity which make provides for longer memories.
We haven’t always gone to MK first morning of any trip because across all age groups Epcot is far and away our favorite park. One trip we had a kid who’d just lost a tooth. We had a great time chatting up CMs in the different countries about tooth fairies .
I wholeheartedly agree that your second family know nothing, especially they won’t know what they’re missing. Add in Genie + to the projected cost. Do as many options as possible - park hopper, water park. With a bigger group, flexibility becomes even more key. As is knowing where every bathroom is. Not very many days into our trip with the 3 girls a new rule was introduced - if anyone goes to the bathroom, everyone goes. At least try!! 
If your trip plans will accommodate park hopper I suggest thinking - and setting up your spreadsheet - of your days in chunks. Morning chunk and afternoon/evening chunk. I submit that the kids are used to getting up and out the door early enough for most rope drops, or close to it. If you’re out the door between 7 and 7:30, and in a park by 8:30, by lunch time you’ve been in the park nearly 4 hours. And everything is stimulating. We usually plan to exit a park by 11:30.
We don’t necessarily nap or swim, sometimes we just cool off. 
Or have quiet for a minute or two. The two older girls liked changing clothes for the evening. We brought glow sticks with us from home for evening gussying up.
By 2:30/3pm your group can be at the bus stop for your afternoon/evening park. Leaving about 7 pm to get kids in bed by 8 gives you more than 3 hours in that park. And nearly 8 hours total that day in parks. More than enough time to be stimulated.
Our first day with these girls did include opening MK. We went first to Adventureland to start the day with Aladdin’s Magic Carpets. All 3 girls screamed in terror while on this ride. Pretty much declaring they would never ride it again. (I feared for any other Disney ride.) We did ride - in some order - Pirates, Splash and Big Thunder Mtn - before heading to Crystal Palace for breakfast with Pooh and friends. The rides were well tolerated - way less scary than a magic carpet. The breakfast was a huge hit - granma had bought autograph books. (Something we never did ) Pretty sure not much was eaten, but the buffet choices made a huge impression. After breakfast we had a FP for a meeting with Tink and then we were headed back to our resort. The girls played Uno in the cool quiet until we dressed and left for our early ADR at Marrakesh. We went directly to the Friendship across to Morocco. Dancing with the belly dancer was so much fun. We walked thru France, UK and China stopping for photo ops in each country. We probably did Spaceship Earth. And, since we were now at the front of Epcot, it was an easy walk to the bus stop (with a bathroom break along the way).
Day 2 was similar, opening AK, Safari, Everest except for the 4 yo, playing on the drums, face painting, photo ops and an impromptu late breakfast at Tusker House. (Waistbands aren’t the only things that benefit from elasticity at Disney). After a resort break we were going to head to Fantasyland for princess time but Hurricane Dorian changed our plans to Dominos delivery and games. We kept to our plan of Typhoon Lagoon for day 3 never managing to incorporate our lost FL time. The girls didn’t miss it. Day 4 was open HS with 11am lunch at 50s Prime Time. Afternoon was girls make over provided by a cousin and aunt and new dresses from grandma for supper at Akershus followed by rides on Frozen and Gran Fiesta as well as photo ops in those countries. Friday was another non park day with an early supper at 1900 Park Fare. Saturday was Epcot - Living Seas and talking with Crush followed by lunch at Garden Grill. Aft/evening was Toy Story Land - great photo ops - and Sci Fi Dine in.
For shirts we had a color of the day. Everyone wore teal on MK day - solid or print, new or old favorite, your choice. Our photo pass photo looks pretty good. We match without huge sameness. Green for AK. White for HS.
Except grandma went berserk with matching shirts - in the color of the day - for the girls. And boy howdy did they get CM attention. Idk where she got the idea to get them matching shirts.
Thank you for putting up with this ultra long read. I included our plan in case it helps provide a jumping off place for your plan. I’m incapable of being brief; I’m happy to try to be concise if I can be of assistance. 
