Congrats! I really like the design/decorating process so that clearly influences my attitude toward approaching the task - but I think you'll enjoy it when you actually get going vs. now when it is just an overwhelming amount of stuff to tackle.
I have now furnished two vacation homes from scratch and refreshed one that we bought fully furnished/equipped and am soon going to be furnishing another rental property if it ever gets built (two beach/two ski homes). It is a little shocking how much you need to fill and empty place and have it be functional for yourself and renters - things like knicknacks and art really add up and places don't feel complete without those touches.
As other PPs have said, how much you'll be renting and at what price point will definitely influence what you put into the house in terms of cost and durability. When i furnished my first place I didn't think we'd ever rent it so I furnished it just for us and with everything I loved, but thanks to not being able to set foot there for over two years during the pandemic, we did end up renting it on a limited basis, and have definitely have suffered some damage and wear and tear that I would not have if I'd furnished with renting in mind. Think hard about how things will wear and don't get fragile/precious things. With a beach house, you'll want hardy stuff anyway as it's inevitable that people sit in wet suits and sand gets everywhere.
If you are going to use a management company to help with your rentals/cleanings/maintence etc. I'd look for that right away as they can be invaluable in helping you navigate outfitting the house - they will have a list of what they consider the necessary items (mainly kitchen but the basic furniture each room should have and other extras renters will expect to have provided). And then from there you can put your own personal touch on it and add things that your family will use and enjoy.
Make sure you have lock offs to hide the more fragile/special things. I have closets at each house where I keep gear (located in the garage or an out building for easy access), nicer kitchen stuff and pantry staples and clothing basics etc. that we leave there that aren't accessible to guests (located in an interior closet). We added locks to certain closets at the already built places and designed them into the ones we built/are building.
I may have a list that we got from our management company for one of our ski condos that I could send you if you're interested. In that case we were leasing the newly built condo back to them for three years so they provided a list of everything we were required to have in the home when the lease commenced. But even the management company for the house we bought furnished and equipped did a walk through and gave us a list of things they felt needed to be added or upgraded.
We do lend our houses to friends but not to just anyone. We enjoy letting people we care about use the places if we aren't there but we limit that use to off peak times (usually) so that we can still maximize rent and to people we trust to care for the places as we would. We haven't been burned yet - friends usually take way better care of the places than renters do and are usually super appreciative. We pass out of pocket cleaning fees through and that's it. We have tiny families so family asking to use it hasn't been a massive problem. We usually can say yes but have had to say they weren't available here and there.
As far as your kids inviting everyone they know - I think that's great! It won't be forever and it's huge that they are so excited to spend time there. We got our first place when the kids were in early middle school/elementary and it has definitely fluctuated over time how much they want to go. Having friends is always a surefire way to get them excited. And we have created wonderful memories of houses full of people enjoying downtime together, so I say embrace it! You'll still get enough time just you and your family, especially as the kids grow up.