Do they hear you praise them to other adults whom they know you respect? I had a friend in college, a grad student who was much older than I was. She had two children (she was a widow who had gone back to school), and I was very taken with the way she praised them to us when the kids could hear. I make sure I do that with mine when she deserves it - I think it's extra-effective because they know you mean it very seriously that way.
orvida, I don't mean to mess in your business or criticize or anything - that was just in response to your comment to avierra that you are proud of them "even if they might think otherwise ... It's just hard for me to show them that they mean the world to me."
(This is one of the things that I hate electronic communications for - you can't see the other person's face so a lot of the emotional context gets lost.)