How do you detect an instance in a part?

As RedCombee has answered, why not use the FindFirstChild Method? If the FindFirstChild method finds the Child, then it'll return true, elseif could not find the Child, then it'll return nil, here's an example of what I mean;

local child = game.Workspace:FindFirstChild("Child") --Here's 'FindFirstChild'; this will check to see if 'Child' is existant in the Workspace

if child then --If 'child' is existant
print("Child found!") --It'll print this in the Output
else --Elseif 'child' is not existant then
print("Child not found!") --It'll print this in the Output
end

Or, you could do FindFirstChild("Child",true), that will check the game, even if 'Child' is in a 'Model' type instance, it'll still beable to tell if existant or not, here's an example of what I mean;

Game
	Workspace
		Model
			Child

Now, let's use the FindFirstChild("Child",true) method;

local child = game.Workspace:FindFirstChild("Child",true)

if child then --If 'child' is existant
print("Child found!") --It'll print this in the Output
else --Elseif 'child' is not existant then
print("Child not found!") --It'll print this in the Output
end

And, as you'll see, even though it's not exactly in the 'Workspace', it'll still beable to find it when you add ,true in with the FindFirstChild method. Hope this helped!