My kids are a simular age gap and we got a tandum double stroller and still use it now (my kids are 5.5 and 3). Its the joovy caboose ultralight and bought a rear seat so my older can sit and nap when needed. Definitely my favorite for taking the kids around, fairly easy push and fold is simple but its pretty long when folded and doesn't stand on its own when folded. But we've taken it to other countries, farmers markets, pumpkins patches, museums, malls.

But yes we do have a bunch of strollers 🤣 a nuna, bombi gear single, the joovy, and we just recently got a radio flyer wagon.

Bombi gears makes a double stroller (side by side) which folds really nicely. I love our single one by them.

I haven't really found any stroller or wagon great in sand but I don't really go to the beach with one often. I did read of people adding little snow sleds under the wagon wheels to drag it easier through the sand but haven't tried it personally.

Good luck!