Hey Liviam sounds great what you have done, I was thinking about the paint and why not just do a feature wall in each childs bedroom??

Another place to check for wall decals is DH gate, its like chinese ebay, dont let that turn you off I have made purchases from there before its not a problem and its safe, just like our ebay check the sellers feedback. You will find the decals on there for really cheap, sometimes in bulk then you could just resell the ones you dont want - just a thought.