Ivy & Ivy-Rose - I don't know why but I hate the name Ivy, makes me think of a miserable mean old lady. Ivy-Rose is a bit nicer, I like hyphenated first names. Poison Ivy too.
Dahlia - I reallly love this, gorgeous plant, all the varieties. Would you pronounce it Dar or Day?
Violet & Violet-Rose - Like Ivy, I hate Violet, though it seems very trendy right now. Violet reminds me of a really awful nasty bully from a book written in the 70's or something, no idea why, just think it's horrible.
Azalea - I like it, interesting nice sound, doesn't conjure up anything horrible for me!
Amaranth - bit odd, I don't know this plant, bit clunky sounding, looks masculine to me. might get shortened to Amy or Ammy.
Can I suggest - Zinnia, Marigold, Aster, Clementine.
Boys is tricky, personally I'm not a fan of flowery names for boys like Rowan etc. There are bird names like Robin or Wren but personally I don't like them either, they feel a bit 'wet' for a boy. Monty is a good one though, or Don?!
Have you come to any decision? Good luck and congratulations!