** Beds stay in place for a long time, so flat-packing for a move is unimportant.
** Box springs are unnecessary and cost a few hundred
** Whatever the mattress rests on gets hidden by a spread
** A footboard serves no structural function
** The headboard does not have to be attached to the frame, and if it's separate, there's unlimited design options
** Should be kid proof and allow for storage underneath
The approach I've taken on about a dozen examples is to construct a simple platform out of 2x6 framing lumber and OSB, dimensioned to the size and thickness of the mattress to stand at the desired height (24" for me). It can be built in the morning and slept on that night. Headboards have been wood, and once an iron gate, attached to the wall with cleats or even lagged into the studs.
A single bed on one frame, and queens and kings on a pair of frames