First of all: WOW! What an amazing design on the building itself! It's beautiful.
Secondly, thanks for the compliment about my build. I always battle with interior design. When I try a neutral palette, it looks drab and boring. When I try brighter colours, it gives me a headache. When I try darker colours, I get depressed. When I try to imitate builds like Odinsdottir and Agneza; it just looks stupid. So now I mix and match and fool around until I get a colour scheme I can live with, and that's how it stays because by the time I'm done I'm so sick of it, I have a permanent 'Nauseous' moodlet.
This is the first build I've done that I can honestly say, I would live there myself, probably because the mismatched clutter of stuff mimics my house perfectly. Lol.
Finally, onto your build: As mentioned above, I'm not fantastic at interior design. First thing I would do though, is to look at your nearest mall for ideas. Next, the toilets, I would change to depict genders for each. Peach/Pink/Cream for the ladies, gents maybe blues or darker browns. Keep the plumbing and fixtures neutral and change the flooring style to tiles in those colours, and maybe half tiled walls.
The gym, I would use wooden floors instead. Why is there a fridge/kitchen in the gym area? Possibly change to showers instead? Also not sure about the table-soccer game?
Library, remember a children's section. A 'back-2-school' bookcase if you have one, a blocks table and maybe a couple of those puzzle carpet floor tiles?
The movie theatre, could you somehow put the back row of seats on a platform so they're raised higher than the ones in front?
The supermarket, again, I would tile or use linoleum on the floors, shuffle the tables around to create aisles and put the beauty table by the window.
The dayspa and restaurant, I'd add some decorative planters.
And the bookstore... Again I'd form aisles with bookshelves, add some piles of books if you have any under decor, a toybox or two, and the blocks table and puzzle flooring and some kids wall decor for the toy area.
Unfortunately I can't really help with colour schemes and layouts. The above ideas are just things I like to see when I go shopping. Hopefully others will be more helpful.