A day and an afternoon/evening is definitely short but with park hoppers, LLMP and knowing your priorities of rides ahead of time, you’ll be in the best position to get the most of it.
I would definitely get LLMP for the full day, and for the partial day, wait until you arrive at DLR and first check what availability there is for any LL’s before deciding on purchasing it.
Based on the hours from last year at that time, it looks like they could go as late as 10pm for DCA and midnight for DL. I highly recommend taking full advantage of as late as you can for those hours because they can be just as glorious as early.
With teens I would definitely start that afternoon in DCA and go for a couple of the rides they are most excited for that are unique to DCA that don’t have prohibitive waits (or if you bought LLMP, ones that you can still get LL’s for in the later afternoon). Incredicoaster or GotG:MB are favorites of my teen/tween kids, and ones they love repeat rides of. Goofy’s Sky School is also one they like but is basically a wild mouse coaster so nothing especially unique to DLR, but fits the bill of some thrill and fun. Radiator Springs Racers is also pure fun (not really high thrills) but it’s definitely a must-do (it won’t be an LLMP option but does have a good single rider line & the LLSP isn’t a terribly high price for one shot at riding it together without a 60+ min wait).
For dinner in DCA, Wine Country is good choice with the ambiance and menu. I also would strongly recommend taking a look at the excellent quick service options in DCA to have more time doing things in the parks. The San FranSokyo area has awesome options that are readily available on mobile order (the seating may be tricky if you eat at peak mealtimes though because there’s never enough seats). We also love Award Weiner’s and and Carsland also has some great options between Flo’s and the Cozy Cones, plus you could time it to be in Carsland to see the neon lighting at sunset that’s a quick option to catch some of the fun ambiance/entertainment without spending a lot of time away from rides.
Then I would definitely hop to DLR after dinner and choose an area you’re most excited about to knock out some rides but be warned that any time you want to cross around the park you’ll have to navigate busy walk-ways due to all the nighttime entertainment offerings. You’ll either want to stick to the “West Side” (Adventureland/New Orleans Square/Pooh Corner) and use Galaxy’s Edge as your pathway to get to anywhere closer to the hub to Fantasyland and also if Fantasmic is scheduled, avoid the Rivers of America and the alleys of New Orleans Square 15 min before and after the show OR stick to the “East Side” in Tomorrowland/Fantasyland/Toontown but generally avoid crossing the parade route especially towards the hub/Main St.
If you watch one nighttime entertainment, I would highly recommend it being the projections from either IASW or Rivers of America for Wondrous Journeys or catching a later showing of the Paint the Night Parade.
If you really don’t want to catch any entertainment though, that can be a good time to knock out some popular rides. If they are not doing fireworks then all the Fantasyland dark rides around the castle will stay open (when they do fireworks they have to close anything in the same building structure as the castle - Snow white, Pinocchio, Peter Pan, Mr. Toad’s & the carousel because they launch smaller bursts from that build, and depending on the wind direction, possibly even those directly around the castle have to close too (Storybook Land Canal Boats, Teacups, Alice & even Matterhorn).
Then towards the end of that partial day, I would make a final decision on where you want to start your full day at and with you being an onsite hotel guest you’ll have flexibility to change your park reservation since onsite guests have a separate pool of reservations than day ticket holders or Magic Key pass holders. But I would have an idea of which one you would like to start at based on interests. A one day blitz generally is easier to do by starting in DCA but it will definitely depend on what the group’s interests/high priorities are between the 2 parks and you can evaluate that best after the afternoon/evening of experiencing both. Just be sure to change the reservation if you need to before midnight.
And as for what to skip, I tend to skip Galaxy’s Edge when we are in WDW (the park we are at less than DLR) so I would agree that unless you have a major Star Wars fan, it can be skipped. Also, unless they are family favorites you can also skip Toy Story Midway Mania, Winnie the Pooh, and Little Mermaid and Star Tours. These are all exact copies as their counterparts in WDW with inferior or similar queues to their WDW versions.
Soarin’ may be exactly the same if it’s the Around the World version but in March it’s usually switched over to Soarin’ over California for the DCA Food & Wine festival (which was once upon a time at WDW too but not in awhile). Depending on how you feel about the versions of Soarin’ it might be a high priority or totally skippable).
With teens you could also easily skip Jessie’s Critter Carousel, Jumpin Jellyfish, Inside Out Emotional Whirlwind and Golden Zephyr in DCA. And in DL the teens might not highly prioritize (so could easily skip) teacups, carousel, Casey Jr., Autopia, and the toon coaster in Toontown.