For accessories, you can hang pictures of turtles. You can also use wall decors like wall clock, turtle designed-accessories (e.g., wind chimes) for the flooring; it will be simple enough if you can paint it with but with simple patterns of blue or green. You can also use personalized rugs or carpets that show that you are more on the aquatic part of the room.
Another theme that is partnered with turtles is bugs. Basically, this is what the turtles eat so you can use bug accessories in various parts of the room. You can buy these bugs in children’s store for just a cheap price or you can make your own one and bond with your child with it.
Other things that you can use are a basket in which you can put all turtles or your child’s toys in order it won’t be that messy enough, you can also use nature like twigs and glue it on the walls so you really can feel the naturalness of the room.
Now, don’t just overdo things. Remember, simplicity is still the best design. Let your child help so that he or she can feel the ambiance of his or her room. Harmony is also one thing that you should bear in mind. Pick all your designs and make sure that it compliments one another. Have fun designing!