Since you have a wall mount sink, most of the floor will be visible. Go with what you love. The 12x24 will look great and will give you the modern look you're going for.

If your quartz countertop is solid or just has flecks of sparkle, it's nice to bring some pattern in with the floor tile. You can also add some pattern to the floor with the way you lay the tile. Try to find a tile store that also sells quartz so you can coordinate the two. Be aware that you may have to add a new subfloor for the tile, which would make the tile higher than the hardwood in the hallway. This happened in our kitchen when we replaced sheet vinyl with porcelain tile. I do not like the wooden "speed bump" strip between the hardwood and tile. We would have liked to use porcelain tile in our guest bath remodel, but again, it would be higher than the adjacent carpet. We used Armstrong Alterna LVT. There aren't as many color and pattern choices as porcelain tile, but it it has a limestone composite base and is grouted just like real tile, so it's not really vinyl-y. Nice stuff, and a lot cheaper than porcelain tile.

Even though I love the way the hex tile looks, with kids and durability being an issue, I'd pick a larger scale rectified tile (I think that's what it's called), meant for tiny little grout lines. I'd pick a medium tone tile with matching grout so it wouldn't look dirty, and there would be much less grout. Personally, I wouldn't put wood flooring into a small bathroom with little kids. I'd stick with tile.

For continuity, it's the better look. There is no set rule, why use the same color for all three bathrooms? A powder could be really done differently, but if the space is small keeping the floor the same makes sense.