My 2 cents: Go for it.
If a kiddo enjoys math competitions (or wants to try them to see if he'd like them), I'd focus on the positive benefits of the experience and caution against being overly concerned about awards and prizes.
Carol Dweck's Mindset encourages praising effort, talking up the fun of the mental challenge worthy of one's potential, encouraging a growth mindset.
One aspect or application of a fixed mindset is that gifted kids, in order to be seen as "right" or "smart", may stop taking appropriate risks, possibly shunning a challenge and preferring easy work which may represent a level of underachievement. A fixed mindset may work against them and be exhibited as a lack of resilience.
