Maybe use a mix of blues, such as turquoise and navy, along with the white, and the green carpet. These could evoke ocean colors. Not sure if the rug is a true green or an avocado ( which may not work). Some nautical or sealife touches can be used such as round porthole look mirrors, model sailboat, curtains with seashells or fish, etc.

If you are in Sydney try Grandchester Designs. I have the same problem.

Our 2 kids are 6 and 3. They will have bedrooms 2 and 4. A 3rd child is on the cards in near future, but will be downstairs with us for a good while. So don't need bunk beds (unless they want them when a bit older), would be a single or king single in each room for years to come and maybe double down the track. I have realised that the 'extra' window in each room will be a highlight window, for extra lighting and cross vent. Window coverage will *probably* be plantation shutters.

What an exciting project - providing you have adequate ceiling height and the budget to pay for the services for a bespoke solution. You may want to consider using one of the better shop fitters who can design, build and project manage the whole job. They are used to doing quirky, small scale projects to a high level of detail. The better ones don't come cheap as you are paying for a multi-disciplinary service that is project managed and co-ordinated.