How to find one target out of all children selected?

You're already finding one target out of the table. Just not the one you want it to be.

Your code needs some polishing; you have the right idea with magnitude though. It should find the nearest target first by comparing distances with magnitude then return that target so you can do your firing code.

The for loop was only returning the last enemy in the table who exists and was alive. I've put some variables in the code to give you some ideas so you only get the nearest target.

local enemies = {"Bob","Tom","Jerry","Marshal"}

local function FindNearestTarget(targetTable)
	local target = nil 
	local MAX_SEARCH_DIST = 500 -- How many studs away do you want to search for
	local currentDist = 0 -- Compare currentDist with lastDist to see who is closest
	local lastDist = 0 -- The target who's distance was closest 
	
	-- Do your "FindNearestTarget" code here ==========
	for _,enemy in pairs(targetTable) do
        if enemy then -- if Enemy exists 
            --while wait() do -- Don't need this while loop
                repeat --wait() -- Don't need this repeat loop
                    target = enemy --Add something else to find only ONE target, not all children.
                until target ~= nil --and target.Alive.Value == true
                    --local magnitude = (target.Position - towerMain.Position).magnitude
                    --if magnitude <= towerMag.Value then
                        --Fire(target)
                    --end
                --break
            --end
        end
	end		
	-- This for loop will always return the last person in the list
	-- In your case, the last target who exists and is alive
	--====================================================
	
	return target
end

local target = FindNearestTarget(enemies)

while target and wait(1) do
	print("Found a target and his name is: " .. tostring(target))	
	-- Always prints "Found a target and his name is: Marshal"
	if target then
		-- Do firing code outside of your for loop
	end
	target = FindNearestTarget(enemies)
end