Why doesn't the Touched event fire on a player when they are sitting?

Hello, you can use magnitude instead, like this:

Script:

local players = game:GetService("Players")

while wait() do
    for i, v in pairs(players:GetChildren()) do
        local char = v:WaitForChild("Character")
        local foot = char.FOOT -- Idk how it is called right now, you can use leg instance or foot instance on R15
        local lava = lavaLocation -- past here your lava part location

        if (lava.Position - foot.Position).magnitude < 1 then -- if lava is less then 1 foot away from middle of leg or foot part
            char.Humanoid.Health = 0 -- kill player
        end
    end
end