We took a road trip like this when our kids were 2 and 5. It was long, but it was OK. This was 13 years ago at this time of year - we drove from NY to Ohio to visit with family, then to Wash DC then back to NY all in a week. There were no tablets or smart phones at that time - but we spent $280 on a portable video player. The screen was the size of the contemporaty smart phones. BUT that video player was the life-saver of the trip. We had a giant canvas bag full of video tapes and to this day I recall the lines to many of the videos they watched on that roadtrip. We also had a small cooler - we froze water bottles to use as ice and had cheese sticks and yogurt tubes, fruit and snacks like pretzels and goldfish.
Now with smartphones and tablets load up movies and shows on your phones and tablets, get headphones that your kids will use (little ones don't always like earbuds). Don't bring along books or writing for the car part of the trip - that causes car-sickness. Google ahead of time different car games you can play. We were big on the alphabet game (find things outside the window that begin with letters as you go through the alphabet - it was always easy as we went through NYC - difficult in the farmlands of PA), make up progressive stories - you begin the story with two sentences and then it gets passed around the family members as each builds the story with a few more sentences, etc. I'm sure if you google family car trip acitvities you'll find a ton more.
As for the beach - we live a couple of miles from the beach. It's not very relaxing with little kids so as long as you're not expecting to read a book and lounge in the sun you'll be fine. Yes - baby powder is a must to keep in the trunk of the car to de-sand your kids before you get in the car. A mesh or plastic "net" bag for sand toys is good to have. You must have shovels - you can get larger plastic spades in most 5&10's near the beach or even Target I think. They're maybe 2 1/2 feet long, a wooden handle, a plastic shovel part. You need at least two - you can dig a deep hole so your little one has a safe place to play in the water where the waves won't knock her down. You and your 7 yr old can bury daddy in the sand, etc. While you can probably find a set of sand toys at any store within an hour of the shore, you can also use plastic containers from your cabinets. Deli salad containers, ricotta cheese containers, pastic cups, anything that can hold sand. Consider buying a "neat sheet" which is perfect for the beach. Have a small cooler that can hold frozen water bottles & frozen juice boxes. when you put the cooler down for the day at the beach, dig a shallow hole to put it in - the sand is cooler a few inches down. Put the cooler in the shade (shade of yoru chair, a beach towel, an unbrella, etc. Use ziploc bags for bitesized fruit like grapes & cherries. Sand will get on everything so so be prepared. with littel kids the blanket will be sand filled the whole time. Anticipate that. If the sun is hot turn your flip flops upside down when you take them off so they won't be too hot on your feet. If you brush aside the top level of sand where you're standing the lower layer will be cooler to stand on or lay your blanket on. Slather your kdis with sunblock before y0ou leave the house / hotel in the morning. it's so much easier to put if on and do a thorough job while they are naked inside that when the wind is blowing and they'r standing in the sand at the beach. Remember to get the tops of ears, back of neck, forhead, eyebrows, etc. Reapply to shoulder & nose more frequently than other areas.
Stopping along the drive for playgrounds is essential - and the good thing is that with smartphones you can easily find them. 45 minutes in the park with a sandwich or pizza can make all the difference for the kids.
Finally - always have a change of clothes and a container of wipes, and paper towels easily accessible. I have vivid memories of trucks flying by within only a few feet of us while my husband and I scrambled to dig out clothes from suitcases, paper towels and wipes on the side of the highway in order to clean up and change our 2 yr old son who had thrown up (too many goldfish I think) all over his car seat. We still laugh about that and recognize that instance at the watermark of having attained professional parenting status. We split the tasks, we got it done quickly, no one cried and we didn't get mad at eachother. And the car smelled like puke for the rest of the drive!
You will have a blast! Remember to take photos and videos of the silly moments, the every day moments, the pooped out end of day moments. When we look at our old videos we are astonished at how very quickly those days flew past. Savor these times - they are sweet.