Why not just add a giant mouse hole for the kids to crawl through ! Much more fun ! ( and easily blocked up again when they grow)

This kind of alteration really requires an architect and a good contractor. The architect will know all the local requirements and how to meet code on everything. You will want/need to insulate the ceilings of the downstairs for sound/fire control, separate the electrical and plumbing lines, and all the things the average homeowner doesn't know or hasn't thought of. In the long run, an architect will save you money. Your friends who have remodeled may refer you to their architects. It's worth interviewing two or three in your home to see which one can help you achieve the best result.

It's difficult to get on the bed and off it, in a platform bed. Also if you wake up in the night without lights chances are you may hit the platform when getting off or on.

You can get a custom wall art done , glass with bronze, glass painting, that wall should be a center of attraction, anyone walks. I have done this for a site, it works wonders, guest appreciate a unique feature. this can be done as DIY project on weekend also. these are some some real world executed ideas. cheers mate.