Can I GetChildren of only a certain class?

Yes, you can. By using :IsA() you can easily identify what you want from a group of items. For example let's say you had a model with random items:

local Model = script.Parent

for _, item in pairs(Modle:GetChildren()) do
    if item:IsA('Tool') then
        -- do something to the tool
    end
end

Hopefully, this helped. Best of luck developer. Comment if you have problems.

Unfortunately, while this has been requested on the official developer forum, it is not [yet] a feature. You can, however, use a loop containing an if statement to check if an item is a specific class.

local newTable = {}
local items = something:GetChildren()
for i,v in pairs(items) do
    if v:IsA("Tool") then -- Any class can go here.  You can also put multiple statements.
        table.insert(newTable,v)
    end
end
-- do something with newTable here.

Use :IsA(className) to check the class of an instance.

function GetChildrenOfType(parent, t)
    children = {}
    for i, v in pairs(parent:GetChildren()) do
        if (v:IsA(t)) then
            table.insert(children, v)
        end
    end
    return children
end

At the moment we are not able to only GetChildren of one class. This can be achieved in a few ways but the best way would be using a generic for loop to add all children of that class to a table.

childrenOfClass = {} -- Table that the children will be put in.
for _, v in ipairs(parent:GetChildren()) do -- Loops through ALL children
    if v:IsA("Class") then -- Checks if child is of a specific class. 
        table.insert(childrenOfClass, v) -- Adds child to table
    end
end

You could make this more useful by turning it into a function like this:

function GetChildrenOfClass(parent, class)
    children = {} -- Table of children
    for _,v in ipairs(parent:GetChildren()) do -- Loops through ALL children
        if v:IsA(class) then -- Checks if "v" is of class "class"
            table.insert(children, v) -- Adds "v" to table
        end
    end
    return children -- Sends the children back to the line that called it.
end

If my answer solved your problem, please remember to mark it as correct!