I've never made a quilt for an adult using blocks designed for a child, but I have restored a fair number of old quilts and made some quilts using older blocks. For me, the key thing is to use setting and border fabrics that are compatible with the blocks. Also, the overall design of the quilt should be consistent with the era the blocks are from.
If you could find several printed feed sacks with the same pattern, I think it would be nice to use those for sashing or setting blacks. You could also use reproduction fabric. You mentioned the quilt is for a king size bed; I think you'll have to use wide borders.
I am looking forward to seeing your pics!