How to Find all Children of an Instance by Class?

That I'm aware of, there is no built-in function to do it. The best way to get all children by a class is writing a function for it yourself, which is fairly simple!

Loop through the instance's children, check their class, if it matches the one you are looking for, then add it to a table. In the end, return the table.

It's fairly simple to write

local function getChildrenOfClass(instance, class)
    -- table to hold the children
    local children = {}

    -- looping through the instance's children
    for i,v in pairs(instance:GetChildren()) do

        -- check if the child's class matches the one we're looking for
        if(v.ClassName == class)) then
            -- we add it to the table
            table.insert(children, v)
        end
    end

    -- we return the table, so that it can be used when calling the function
    return children
end

Afterwards, you can call your function whenever needed, and access it!

To access it, you can do it in the same way that you would for FindFirstChildOfClass() !

-- This will allow you to use the children you've selected using the function
local childrenOfClass = getChildrenOfClass(model, "Part")

for i,v in pairs(childrenOfClass) do
    print(v.Name) -- This would output the part's name, you can do anything else with it!
end

Documentation for tables, ClassName, loops

function GetChildrenOfClass(Instance, Class)
    local ChildrenOfClass = {}
    for _, Child in pairs(Instance:GetChildren()) do -- iterate through table returned by GetChildren
        if Child.ClassName == Class then -- Check if ClassName matches Class Argument
            table.insert(ChildrenOfClass, Child) -- insert into table
        end
    end
    return ChildrenOfClass -- Return the table
end