The problem with positive 'reinFORCEments', rewards and praise is that you are still trying to control them - it's subtle, but they know it and so should you be aware. The kid will get fixated on doing things for the end results, killing the joy they can have from simply doing it because it's inherently interesting, fun or challenging.
I used to wonder how praise could be damaging, but think of those A students whose life falls apart when they get a B+ - they've become praise junkies and stop valuing themselves with just a minor blip. On the other hand, I would not try hard at anything because I was frightened of failing, I never felt I would be good enough to be rewarded.
Help your kids learn to seize the day and live in the moment, encourage them to 'do' no matter whether they succeed or fail, this will build their core resilience to what life throws their way - they will learn to pick themselves up and keep going. If life is a journey, we need to be able to enjoy the process of getting 'there'. Death is the inevitable goal and we have no idea whether what's after will be worth waiting for! So let them get on with life and love it for what it is, don't trick them into believing it will be better if they get given special treats! The best you can give them is love whatever happens.
Check out Alfie Kohn's book: