@Tania294, you have a gorgeous house! I would not mind sleeping in a kid’s room for the rest of the house and the pool 😉
actually, I often did it, staying with family and friends. There are some perks: when would a childless person get to read children’s books 😉
more seriously, and after a look at the rooms, I would say, you could do a few things to make it more comfortable for the adult guests.
The boy’s room is more neutral, the girl’s room is awfully pink. And the chairs won’t support an adult man. I would change the pink carpet, get some foldable chairs and a table for the room. Remove all but one (or all) tissue animals from the bed. Maybe get a box or lowbench supporting an adult sitting on it, to put all the others in it. Your kids won’t want them used by guests or chewed by a pet. Put all the books on the lowest shelf, leave the higher shelf and the top of the small bookshelf cupboard empty for their stuff. If you can provide an electric socket there, it’s a good place for a laptop.
You have nothing on the walls - if you want to change decoration often between family use and guests, you could get a system like in a gallery, a bar under the ceiling and thin chains hanging down for paintings or a whiteboard to draw on etc. If you put some (framed) posters or maps on the walls, that’s easily done and can change the whole feeling of a room. Maybe not go all out between Mickey Mouse and nudes, that would be disturbing for the family 😉
I would leave the stickers on the cupboard, it’s the kid’s room first. But I would counteract the effect with a carpet, probably with a strict geometric pattern, something associated with Japanese design. Or decorative objects in that sense, but as you do not have much free space for the guests’ clutter, a carpet or bed spread would be more practical.