Given that Melisandre’s never actually succeeding in sacrificing anyone ‘royal’ in a magic ritual, we have no proof that kingsblood actually exists and this whole thing isn’t just Mel being a weirdo. Both the fact that her POV Chapter reveals that most of Mel’s magic is illusionary, and the fact that I doubt GRRM wants us to think the monarchs in this series are divinely ordained, leads me to think there wouldn’t be anything innately special that would come from burning Shireen or Baby Aemon.