A boat that sinks is the lesson
Build a Boat for Treasure starts on a dock. Each team gets a plot, a stack of blocks and a river full of rocks, falls and hazards between them and a treasure chest. You build something, launch it, and watch where it comes apart. Then you go back and build again.
That loop is the whole game, and it is a good one. A boat that tips over was top-heavy. A hull that cracked on the first rock needed stronger blocks. Kids figure this out by looking at their own wreckage, which sticks better than any explanation. The stages are grouped green, yellow, red and purple by difficulty, and gold is paid out for each stage a build clears.
The blocks go well past boats
Gold buys chests, and chests hold new blocks: wood and stone at first, then titanium, hinges, cannons and jet turbines. Players use them to build planes, cars and machines that walk. Some players skip the river entirely and treat the plot as a building space, the way a kid might use Minecraft in creative mode.
Up to 7 players share a server, split into color teams. A team can pool its blocks into one shared build, which makes this one of the better Roblox games to play next to your kid on the same team.
Where the Robux come in
The game opened in 2016 from Chillz Studios and has passed 5 billion visits. It is free, and gold from sailing covers everything a kid needs. The shop still offers plenty for Robux: a 500-Robux Gold Multiplier pass, three 250-Robux character passes, and block packs such as a 500-Robux jet turbine set. Chests themselves cost gold and hand out a random set of blocks, with the odds listed in the game.
For a kid who likes to build, it sits with Theme Park Tycoon 2 near the top of what Roblox offers. If they want the same build-test-fix loop without Roblox's chat and shop, Poly Bridge and Besiege are worth a look. More on the platform itself in the Roblox parental controls guide.