How to check if any instance inside a folder is touched?

TL;DR

Use ipairs loop:

--KillBricks.Touched:Connect(respawn) --I want it when one of the bricks inside KillBricks folder touch this event triggers

for _, KillBrick in ipairs(KillBricks:GetChildren()) do
	KillBrick.Touched:Connect(respawn)
end

Explanation

You need to use a thing called generators or iterators or just loops, a brief description of iterators is "they will repeat code block while specified function does not return nil", in your case you will use it to repeat a code while next child exists. Generators are created the same as normal functions:

local function generate()
end

For generators there is a built-in syntax that looks like this:

for ... in generator_function do
	-- some code
end

Where ... stands for parameters returned by the generator, the code between do and end can be called an iteration, while generator is running it will repeat this code forever in a loop. Generators should return a value, let's return 5 as an example:

local function generate()
	return 5
end

for number in generate do
	print(number)
end

This will print number 5 repeatedly until it breaks, for me it printed the number 4996 times. They way this worked is that it called the generate function, the function returned 5 so number variable held value of 5, it was not nil so the iteration code was ran which was just print(number), after the iteration finished, it repeated the same action, it called generate again, the function returned 5, number held value 5 and the iteration code began running, this is an infinite loop. This was bad way of making a generator, let's make a generator that will loop over all children in the KillBricks folder, using Instance.GetChildren you can get list of them. To create a generator for this task, you need to know at which position in the list you are, return a child at that position, run the iteration function and increment the position, repeat that until position is out of the children list bounds (means all children were looped through). A good way to create this is to make function which returns generator, this will let you create local variables inside the generator:

local function iterateChildren(instance)
	-- list of all the children of the instance (all the killbricks for your case)
	local children = instance:GetChildren()
	-- current position of the list, every element in list
	-- has its own position, in this case all the children
	-- in the list have ordered position so we can
	-- increment the position to get the next child from
	-- the list
	-- first position is always 1, the reason why we start at
	-- 0 is because later we increment the position by 1
	-- and we need to increment it before we call "return"
	local index = 0

	-- this function is the actual generator that will be repeated
	-- every iteration this function will increase position to
	-- get the next child and return a value at that position
	-- from the children list, then the "do" "end" code will run
	-- where the parameter will be the next child
	return function()
		index += 1
		return children[index]
	end
end

local KillBricks = game.Workspace.KillBricks

for KillBrick in iterateChildren(KillBricks) do
	print(KillBrick)
	-- :Connect the function for currently iterated brick
	KillBrick.Touched:Connect(respawn)
end

Cool generator, but since this is used very often, Roblox has a built-in generator that will do this thing for you, it's called pairs and ipairs, first one returns generator that loops through all elements in table and returns current position and the value at that position for the iteration, position can also be a key, example:

local t = {
	a = 5, -- key a
	b = 100, -- key b
	c = "Hello", -- key c
	d = 2000, -- key d
	[1] = 200, -- position 1
	[2] = 300, -- position 2
	[4] = 30000 -- position 4
}

for key, value in pairs(t) do
	print(key, value)
end
--> a 5
--> 2 300
--> c "Hello"
--> b 100
--> d 2000
--> 4 30000
--> 1 200

You may see that its order is randomized, this is not true for ipairs which goes strictly by order and only works for positions, this makes it faster:

local t = {
	a = 10,
	20, -- position 1
	30, -- position 2
	[4] = 600 -- position 4, notice how i skipped position 3
}

for index, value in ipairs(t) do
	print(index, value)
end
--> 1 20
--> 2 30

a was straight ignored and it stopped at position 2 since it could not find position 3. Since :GetChildren returns table in ordered form, ipairs is the best fit in your case.

Sources

ipairs and pairs

some post about generators

roblox tutorial about loops that also includes pairs and ipairs