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These two are constantly grouped together as "the big kids' gaming platforms," but they work on fundamentally different economic models — understanding that difference matters more for a parent buying currency than any feature comparison.
Roblox: an ongoing platform economy, not a single game
Roblox Gift Card or direct Roblox Robux top-ups fund Robux, the currency used across thousands of independently developer-made experiences within Roblox — outfits, in-experience upgrades, and access passes for specific games. Because Roblox is really a platform hosting many different games rather than one single game, spending patterns can add up faster than a parent might expect, since each individual experience has its own separate in-game purchases layered on top of the base Robux economy.
Minecraft: mostly a one-time purchase, with optional Minecoins
A Minecraft Gift Card / Minecoins funds Minecraft's own marketplace — skins, texture packs, and community-made maps and mini-games — but the core game itself is a one-time purchase, not an ongoing subscription or currency-driven progression system. Spending in Minecraft is fundamentally optional cosmetic and content extension on top of a game that's already complete without any further purchase.
Setting spending limits works differently on each platform
Roblox supports parental controls and spending limits configurable directly in account settings, worth setting up before handing a child an account with a loaded Robux balance — given how many separate in-experience purchase opportunities exist across the platform. Minecraft's more contained marketplace structure means there's inherently less ongoing spending surface to manage once the base game and Minecoins purchase is made.
Robux specifically is worth budgeting carefully
Because Robux gets spent across many independently priced in-experience items rather than one central store, it's easy for a loaded balance to disappear faster than expected across several different experiences a child jumps between — worth having a conversation about budgeting a specific Robux balance toward one or two experiences rather than spreading it thin across many.
Both platforms are genuinely playable without spending
Neither platform requires ongoing purchases to be fully enjoyable — Minecraft is complete after the initial purchase, and the vast majority of Roblox experiences are playable for free, with Robux purchases being optional cosmetic or convenience additions rather than a requirement to participate.
Content moderation approaches differ between the two platforms
Both platforms have moderation systems for user-generated content, but given Roblox's much larger volume of independently developer-made experiences, moderation consistency naturally varies more across that platform than within Minecraft's more centrally curated marketplace — worth reviewing each platform's current parental control and content filtering options directly rather than assuming identical oversight.
Both platforms also support device-level screen time controls separate from their own in-app settings, worth layering on top of platform-specific controls for a more complete picture of a child's overall usage across devices.
The bottom line
Roblox is an ongoing platform with many separate in-experience purchase opportunities, worth setting spending limits for. Minecraft is largely a one-time purchase with a more contained optional marketplace. Both, plus other kid-friendly titles, are in Gaming Gift Cards.