Campervan family here. We bought a Mazda Bongo (pop-up roof, no other facilities) when our kids were 4 and 1. Upgraded to a factory-fit Cali a few years later, and we still have it. Kids are 16 and 14 now.
We've used it for all sorts over the years. Some random nights away just for fun, some trips to the beach (fridge is handy), some weekends (but as a PP said, not as many as we thought as life gets busy), sometimes a parent has taken it alone (husband has used it for biking trips, I've used it to bunk over at a family wedding that I went to alone, plus we've done most of our summer family holidays in it.
Are you sure you are not having more kids? Factory fit Cali is only a four-seater. If you want a third, you'll need sth different.
It's not that easy to be spontaneous. As a family, you generally need a campsite. It's surprising how booked up they can be at weekends and school holidays.
When my eldest hit teens, she didn't want to sleep up top with her sister any more, and she now has a pup tent. Which we can only use at campsites. We sometimes do "transit nights" on longer journeys where we don't set up full camp; then we all bunk in together.
It's taken us a long time to get right, but we've managed three week holidays with bikes and paddleboards.
Get a large awning for sun/rain protection. But then you could say you might as well just camp in a tent. But I like the comfort of the van.
Car seats are the biggest PITA when the kids are young. Take bin bags so you can put them outside, protected.
We've loved our family van holidays, and the kids still like holidaying with us.
We've said many times that if we didn't use the van for our main holidays, it wouldn't be worth owning. It's been our second vehicle.
HTH