I’ve just finished my 2nd kids bed and my 3rd bed (our bed is also homemmade).
The kids beds, whilst not as high a structure as yours are essentially kid size beds at normal ikea kid bed heights but with panels all around with a ntch cut out for entry.
For the panels I used cheap pine shelving boards. 300mm deep, approx 18mm thick 2m lengths about 8 euros per ‘shelf’. Whilst we’re in France I know the likes of B and Q do something similar. Think you can get them upto 600mm wide Even though they’re made of many short sections glued and compressed together they are smooth.
The first bed I stained in blue and shows the wood effect nicely, the 2nd one I’ve just finished has just a linseed oil finish and looks great. Can send you some photos if you want. I can’t be bothered with hosting them on flickr etc.
Birch ply would be the nicest finish possibly for larger widths. But pricey.