It’s highly unlikely that what you have described would break Roblox’s terms of use, but consider these clauses:
§ 4(c) - Acquisition and Use of Robux
“You may not use, acquire, or distribute Robux or Virtual Content except through the Services and except as expressly allowed by Roblox under these Roblox Terms.”
§ 4(d) - Earning Robux
“Creators are only allowed to earn Robux in complete compliance with the Roblox Terms of Use, and Roblox reserves the right to determine whether Robux are Earned Robux in its sole discretion.”
Robux must be distributed through official channels (§ 4(c)). Competitions are not explicitly mentioned, but unapproved Robux distribution might violate the monetization rules (§ 4(d)).
TLDR; as long as you are distributing Robux through official channels, you should be fine.