How do I make other players besides me invisible?

Alright, let me explain what you're having a problem with, there are a couple

for i,Child in pairs(Character:GetChildren()) do
	if Child:IsA('Part') or Child:IsA('MeshPart') then
		Child.Transparency = 1
	end
end

1. All you're doing is getting the Children of the character which that only gets the main body parts and the accessories themselves. In order for that to work, you'd need to get their children aswell

if Child:IsA('Accessory') then
	Child.Transparency = 1
end

1. Accessories do not have a transparency themselves... Just their BasePart children within it.

local Players = game:GetService('Players')

local function InvisibleCharacter(Character)
	for i,child in pairs (Character:GetDescendants()) do
		if child:IsA('BasePart') or child:IsA('Decal') then
			--child.LocalTransparencyModifier = child.Transparency
			local a = Instance.new('NumberValue')
			a.Value = child.Transparency
			a.Name = 'ItemTransparency'
			a.Parent = child
			child.Transparency = 1
		end
	end
	print(Character.Name,'Is now invisible')
end

local function VisibleCharacter(Character)
	for _,child in pairs (Character:GetDescendants()) do
		local originalTransparency = child:FindFirstChild('ItemTransparency')
		if originalTransparency then
			child.Transparency = originalTransparency.Value
			originalTransparency:Destroy()
		end
	end
	print(Character.Name,'Is now visible')
end

local function CharacterAdded(Character)
	wait()
	InvisibleCharacter(Character)
end

Players.PlayerAdded:Connect(function(Player)
	if Player.Character then
		CharacterAdded(Player.Character)
	end
	Player.CharacterAdded:Connect(CharacterAdded)
end)

Alright, what I have done here is set it up for you.. I want you to learn from this... so

What this does is it gets their character whenever it's added and gets the Descendants of Character(Checking if it's a basepart or Decal) and saves all of it's current transparency under a number value named 'ItemTransparency' which we call back later when we want to revert the process(Making the character visible again)

you make the character Invisible by calling function InvisibleCharacter() -- with the argument being the player.Character

you make the character Visible again by calling function VisibleCharacter() -- again with the argument being the player.Character

All the Visible function does is loops through the descendants like before, but instead of checking if it's a basepart or not, it goes through looking for that 'ItemTransparency' Number value we added in before and set the part/decals transparency back to what it previously was set to.

If you still have any questions, feel free to ask! Hope this helps :)